diff --git a/config.c b/config.c
index 67a018fe..09c77d57 100644
--- a/config.c
+++ b/config.c
@@ -1751,6 +1751,21 @@ void setup_config_box(struct controlbox *b, int midsession,
 			 HELPCTX(connection_username),
 			 dlg_stdeditbox_handler, I(offsetof(Config,username)),
 			 I(sizeof(((Config *)0)->username)));
+	    {
+		/* We assume the local username is sufficiently stable
+		 * to include on the dialog box. */
+		char *user = get_username();
+		char *userlabel = dupprintf("Use system username (%s)", user);
+		sfree(user);
+		ctrl_radiobuttons(s, "When username is not specified:", 'n', 4,
+				  HELPCTX(connection_username_from_env),
+				  dlg_stdradiobutton_handler,
+				  I(offsetof(Config, username_from_env)),
+				  "Prompt", I(FALSE),
+				  userlabel, I(TRUE),
+				  NULL);
+		sfree(userlabel);
+	    }
 
 	    s = ctrl_getset(b, "Connection/Data", "term",
 			    "Terminal details");
diff --git a/doc/config.but b/doc/config.but
index 96f5dbe4..27704ee9 100644
--- a/doc/config.but
+++ b/doc/config.but
@@ -1788,6 +1788,22 @@ it explicitly every time. (Some Telnet servers don't support this.)
 
 In this box you can type that user name.
 
+\S{config-username-from-env} \q{Use of system username}
+
+\cfg{winhelp-topic}{connection.usernamefromenv}
+
+When the previous box (\k{config-username}) is left blank, by default,
+PuTTY will prompt for a username at the time you make a connection.
+
+In some environments, such as large corporate networks with \i{single
+sign-on}, a more sensible default may be to use the name of the user
+logged in to the local operating system (if any). This control allows
+you to change the default behaviour.
+
+The current system username is displayed in the dialog as a
+convenience. It is not saved in the configuration; if a saved session
+is later used by a different user, that user's name will be used.
+
 \S{config-termtype} \q{\ii{Terminal-type} string}
 
 \cfg{winhelp-topic}{connection.termtype}
diff --git a/putty.h b/putty.h
index 20cff543..b372d4a3 100644
--- a/putty.h
+++ b/putty.h
@@ -472,6 +472,7 @@ struct config_tag {
     char ttymodes[768];		       /* MODE\tVvalue\0MODE\tA\0\0 */
     char environmt[1024];	       /* VAR\tvalue\0VAR\tvalue\0\0 */
     char username[100];
+    int username_from_env;
     char localusername[100];
     int rfc_environ;
     int passive_telnet;
@@ -790,6 +791,7 @@ void random_destroy_seed(void);
  */
 Backend *backend_from_name(const char *name);
 Backend *backend_from_proto(int proto);
+int get_remote_username(Config *cfg, char *user, size_t len);
 char *save_settings(char *section, Config * cfg);
 void save_open_settings(void *sesskey, Config *cfg);
 void load_settings(char *section, Config * cfg);
diff --git a/rlogin.c b/rlogin.c
index c9e7c5ec..c392fa0a 100644
--- a/rlogin.c
+++ b/rlogin.c
@@ -182,12 +182,14 @@ static const char *rlogin_init(void *frontend_handle, void **backend_handle,
     {
 	char z = 0;
 	char *p;
+	char ruser[sizeof(cfg->username)];
+	(void) get_remote_username(cfg, ruser, sizeof(ruser));
 	sk_write(rlogin->s, &z, 1);
 	sk_write(rlogin->s, cfg->localusername,
 		 strlen(cfg->localusername));
 	sk_write(rlogin->s, &z, 1);
-	sk_write(rlogin->s, cfg->username,
-		 strlen(cfg->username));
+	sk_write(rlogin->s, ruser,
+		 strlen(ruser));
 	sk_write(rlogin->s, &z, 1);
 	sk_write(rlogin->s, cfg->termtype,
 		 strlen(cfg->termtype));
diff --git a/settings.c b/settings.c
index 26926769..7ecd5610 100644
--- a/settings.c
+++ b/settings.c
@@ -75,6 +75,25 @@ Backend *backend_from_proto(int proto)
     return NULL;
 }
 
+int get_remote_username(Config *cfg, char *user, size_t len)
+{
+    if (*cfg->username) {
+	strncpy(user, cfg->username, len);
+	user[len-1] = '\0';
+    } else {
+	if (cfg->username_from_env) {
+	    /* Use local username. */
+	    char *luser = get_username();
+	    strncpy(user, luser, len);
+	    user[len-1] = '\0';
+	    sfree(luser);
+	} else {
+	    *user = '\0';
+	}
+    }
+    return (*user != '\0');
+}
+
 static void gpps(void *handle, const char *name, const char *def,
 		 char *val, int len)
 {
@@ -316,6 +335,7 @@ void save_open_settings(void *sesskey, Config *cfg)
     write_setting_s(sesskey, "ProxyTelnetCommand", cfg->proxy_telnet_command);
     wmap(sesskey, "Environment", cfg->environmt, lenof(cfg->environmt));
     write_setting_s(sesskey, "UserName", cfg->username);
+    write_setting_i(sesskey, "UserNameFromEnvironment", cfg->username_from_env);
     write_setting_s(sesskey, "LocalUserName", cfg->localusername);
     write_setting_i(sesskey, "NoPTY", cfg->nopty);
     write_setting_i(sesskey, "Compression", cfg->compression);
@@ -584,7 +604,8 @@ void load_open_settings(void *sesskey, Config *cfg)
     gpps(sesskey, "ProxyTelnetCommand", "connect %host %port\\n",
 	 cfg->proxy_telnet_command, sizeof(cfg->proxy_telnet_command));
     gppmap(sesskey, "Environment", "", cfg->environmt, lenof(cfg->environmt));
-    gpps(sesskey, "UserName", get_username(), cfg->username, sizeof(cfg->username));
+    gpps(sesskey, "UserName", "", cfg->username, sizeof(cfg->username));
+    gppi(sesskey, "UserNameFromEnvironment", 0, &cfg->username_from_env);
     gpps(sesskey, "LocalUserName", "", cfg->localusername,
 	 sizeof(cfg->localusername));
     gppi(sesskey, "NoPTY", 0, &cfg->nopty);
diff --git a/ssh.c b/ssh.c
index d30626dc..9f2d0111 100644
--- a/ssh.c
+++ b/ssh.c
@@ -3361,7 +3361,8 @@ static int do_ssh1_login(Ssh ssh, unsigned char *in, int inlen,
 
     fflush(stdout); /* FIXME eh? */
     {
-	if (!*ssh->cfg.username) {
+	if (!get_remote_username(&ssh->cfg, s->username,
+				 sizeof(s->username))) {
 	    int ret; /* need not be kept over crReturn */
 	    s->cur_prompt = new_prompts(ssh->frontend);
 	    s->cur_prompt->to_server = TRUE;
@@ -3386,9 +3387,6 @@ static int do_ssh1_login(Ssh ssh, unsigned char *in, int inlen,
 	    memcpy(s->username, s->cur_prompt->prompts[0]->result,
 		   lenof(s->username));
 	    free_prompts(s->cur_prompt);
-	} else {
-	    strncpy(s->username, ssh->cfg.username, sizeof(s->username));
-	    s->username[sizeof(s->username)-1] = '\0';
 	}
 
 	send_packet(ssh, SSH1_CMSG_USER, PKT_STR, s->username, PKT_END);
@@ -7295,7 +7293,8 @@ static void do_ssh2_authconn(Ssh ssh, unsigned char *in, int inlen,
 	     * with change_username turned off we don't try to get
 	     * it again.
 	     */
-	} else if (!*ssh->cfg.username) {
+	} else if (!get_remote_username(&ssh->cfg, s->username,
+					sizeof(s->username))) {
 	    int ret; /* need not be kept over crReturn */
 	    s->cur_prompt = new_prompts(ssh->frontend);
 	    s->cur_prompt->to_server = TRUE;
@@ -7323,8 +7322,6 @@ static void do_ssh2_authconn(Ssh ssh, unsigned char *in, int inlen,
 	    free_prompts(s->cur_prompt);
 	} else {
 	    char *stuff;
-	    strncpy(s->username, ssh->cfg.username, sizeof(s->username));
-	    s->username[sizeof(s->username)-1] = '\0';
 	    if ((flags & FLAG_VERBOSE) || (flags & FLAG_INTERACTIVE)) {
 		stuff = dupprintf("Using username \"%s\".\r\n", s->username);
 		c_write_str(ssh, stuff);
diff --git a/telnet.c b/telnet.c
index 4f0a36fe..20ccc83e 100644
--- a/telnet.c
+++ b/telnet.c
@@ -466,29 +466,33 @@ static void process_subneg(Telnet telnet)
 		    b[n++] = *e++;
 		e++;
 	    }
-	    if (*telnet->cfg.username) {
-		b[n++] = var;
-		b[n++] = 'U';
-		b[n++] = 'S';
-		b[n++] = 'E';
-		b[n++] = 'R';
-		b[n++] = value;
-		e = telnet->cfg.username;
-		while (*e)
-		    b[n++] = *e++;
+	    {
+		char user[sizeof(telnet->cfg.username)];
+		(void) get_remote_username(&telnet->cfg, user, sizeof(user));
+		if (*user) {
+		    b[n++] = var;
+		    b[n++] = 'U';
+		    b[n++] = 'S';
+		    b[n++] = 'E';
+		    b[n++] = 'R';
+		    b[n++] = value;
+		    e = user;
+		    while (*e)
+			b[n++] = *e++;
+		}
+		b[n++] = IAC;
+		b[n++] = SE;
+		telnet->bufsize = sk_write(telnet->s, (char *)b, n);
+		logbuf = dupprintf("client:\tSB %s IS %s%s%s%s",
+				   telopt(telnet->sb_opt),
+				   *user ? "USER=" : "",
+				   user,
+				   *user ? " " : "",
+				   n == 6 ? "<nothing>" :
+				   (*telnet->cfg.environmt ? "<stuff>" : ""));
+		logevent(telnet->frontend, logbuf);
+		sfree(logbuf);
 	    }
-	    b[n++] = IAC;
-	    b[n++] = SE;
-	    telnet->bufsize = sk_write(telnet->s, (char *)b, n);
-	    logbuf = dupprintf("client:\tSB %s IS %s%s%s%s",
-			       telopt(telnet->sb_opt),
-			       *telnet->cfg.username ? "USER=" : "",
-			       telnet->cfg.username,
-			       *telnet->cfg.username ? " " : "",
-			       n == 6 ? "<nothing>" :
-			       (*telnet->cfg.environmt ? "<stuff>" : ""));
-	    logevent(telnet->frontend, logbuf);
-	    sfree(logbuf);
 	}
 	break;
     }
diff --git a/windows/winhelp.h b/windows/winhelp.h
index fc0b8370..4a3c6aaa 100644
--- a/windows/winhelp.h
+++ b/windows/winhelp.h
@@ -1,6 +1,10 @@
 /*
- * winhelp.h - define Windows Help context names. These match up with
- * the \cfg{winhelp-topic} directives in the Halibut source.
+ * winhelp.h - define Windows Help context names.
+ * Each definition has the form "winhelp-topic:halibut-topic", where:
+ *  - "winhelp-topic" matches up with the \cfg{winhelp-topic} directives
+ *    in the Halibut source, and is used for WinHelp;
+ *  - "halibut-topic" matches up with the Halibut keywords in the source,
+ *    and is used for HTML Help.
  */
 
 /* Maximum length for WINHELP_CTX_foo strings */
@@ -70,6 +74,7 @@
 #define WINHELP_CTX_connection_termtype "connection.termtype:config-termtype"
 #define WINHELP_CTX_connection_termspeed "connection.termspeed:config-termspeed"
 #define WINHELP_CTX_connection_username "connection.username:config-username"
+#define WINHELP_CTX_connection_username_from_env "connection.usernamefromenv:config-username-from-env"
 #define WINHELP_CTX_connection_keepalive "connection.keepalive:config-keepalive"
 #define WINHELP_CTX_connection_nodelay "connection.nodelay:config-nodelay"
 #define WINHELP_CTX_connection_ipversion "connection.ipversion:config-address-family"