diff --git a/cmd-find.c b/cmd-find.c
index 02450d97..67f6dbe8 100644
--- a/cmd-find.c
+++ b/cmd-find.c
@@ -135,7 +135,7 @@ cmd_find_best_client(struct session *s)
 {
 	struct client	*c_loop, *c;
 
-	if (s->flags & SESSION_UNATTACHED)
+	if (s->attached == 0)
 		s = NULL;
 
 	c = NULL;
@@ -159,10 +159,10 @@ cmd_find_session_better(struct session *s, struct session *than, int flags)
 	if (than == NULL)
 		return (1);
 	if (flags & CMD_FIND_PREFER_UNATTACHED) {
-		attached = (~than->flags & SESSION_UNATTACHED);
-		if (attached && (s->flags & SESSION_UNATTACHED))
+		attached = (than->attached != 0);
+		if (attached && s->attached == 0)
 			return (1);
-		else if (!attached && (~s->flags & SESSION_UNATTACHED))
+		else if (!attached && s->attached != 0)
 			return (0);
 	}
 	return (timercmp(&s->activity_time, &than->activity_time, >));
diff --git a/resize.c b/resize.c
index 8ed359b4..1c9694da 100644
--- a/resize.c
+++ b/resize.c
@@ -36,10 +36,6 @@
  *
  * This is quite inefficient - better/additional data structures are needed
  * to make it better.
- *
- * As a side effect, this function updates the SESSION_UNATTACHED flag. This
- * flag is necessary to make sure unattached sessions do not limit the size of
- * windows that are attached both to them and to other (attached) sessions.
  */
 
 void
@@ -79,11 +75,8 @@ recalculate_sizes(void)
 				s->attached++;
 			}
 		}
-		if (ssx == UINT_MAX || ssy == UINT_MAX) {
-			s->flags |= SESSION_UNATTACHED;
+		if (ssx == UINT_MAX || ssy == UINT_MAX)
 			continue;
-		}
-		s->flags &= ~SESSION_UNATTACHED;
 
 		if (lines != 0 && ssy == 0)
 			ssy = lines;
@@ -107,7 +100,7 @@ recalculate_sizes(void)
 
 		ssx = ssy = UINT_MAX;
 		RB_FOREACH(s, sessions, &sessions) {
-			if (s->flags & SESSION_UNATTACHED)
+			if (s->attached == 0)
 				continue;
 			if (flag)
 				has = s->curw->window == w;
diff --git a/server-client.c b/server-client.c
index 84b93272..7c2c0288 100644
--- a/server-client.c
+++ b/server-client.c
@@ -1183,7 +1183,7 @@ server_client_check_focus(struct window_pane *wp)
 	TAILQ_FOREACH(c, &clients, entry) {
 		if (c->session == NULL || !(c->flags & CLIENT_FOCUSED))
 			continue;
-		if (c->session->flags & SESSION_UNATTACHED)
+		if (c->session->attached == 0)
 			continue;
 
 		if (c->session->curw->window == wp->window)
diff --git a/server-fn.c b/server-fn.c
index df4cc5b9..24a489ac 100644
--- a/server-fn.c
+++ b/server-fn.c
@@ -431,7 +431,7 @@ server_check_unattached(void)
 	 * set, collect them.
 	 */
 	RB_FOREACH(s, sessions, &sessions) {
-		if (!(s->flags & SESSION_UNATTACHED))
+		if (s->attached != 0)
 			continue;
 		if (options_get_number (s->options, "destroy-unattached"))
 			session_destroy(s, __func__);
diff --git a/server.c b/server.c
index 26a0ce3e..f125fca0 100644
--- a/server.c
+++ b/server.c
@@ -318,7 +318,7 @@ server_update_socket(void)
 
 	n = 0;
 	RB_FOREACH(s, sessions, &sessions) {
-		if (!(s->flags & SESSION_UNATTACHED)) {
+		if (s->attached != 0) {
 			n++;
 			break;
 		}
diff --git a/session.c b/session.c
index 9b74326a..2075abe5 100644
--- a/session.c
+++ b/session.c
@@ -265,7 +265,7 @@ session_lock_timer(__unused int fd, __unused short events, void *arg)
 {
 	struct session	*s = arg;
 
-	if (s->flags & SESSION_UNATTACHED)
+	if (s->attached == 0)
 		return;
 
 	log_debug("session %s locked, activity time %lld", s->name,
@@ -298,7 +298,7 @@ session_update_activity(struct session *s, struct timeval *from)
 	else
 		evtimer_set(&s->lock_timer, session_lock_timer, s);
 
-	if (~s->flags & SESSION_UNATTACHED) {
+	if (s->attached != 0) {
 		timerclear(&tv);
 		tv.tv_sec = options_get_number(s->options, "lock-after-time");
 		if (tv.tv_sec != 0)
diff --git a/tmux.h b/tmux.h
index a7a74eb4..be342f0b 100644
--- a/tmux.h
+++ b/tmux.h
@@ -942,9 +942,8 @@ struct session {
 	struct hooks	*hooks;
 	struct options	*options;
 
-#define SESSION_UNATTACHED 0x1	/* not attached to any clients */
-#define SESSION_PASTING 0x2
-#define SESSION_ALERTED 0x4
+#define SESSION_PASTING 0x1
+#define SESSION_ALERTED 0x2
 	int		 flags;
 
 	u_int		 attached;