diff --git a/tools/qfvis/include/vis.h b/tools/qfvis/include/vis.h
index 92c304fb5..a0863d4be 100644
--- a/tools/qfvis/include/vis.h
+++ b/tools/qfvis/include/vis.h
@@ -158,6 +158,11 @@ typedef struct threaddata_s {
 	set_pool_t  set_pool;
 } threaddata_t;
 
+typedef struct {
+	set_t      *portalsee;
+	int         clustersee;
+} basethread_t;
+
 extern int numportals;
 extern int portalclusters;
 extern int numrealleafs;
@@ -177,7 +182,7 @@ winding_t *ClipWinding (winding_t *in, const plane_t *split, qboolean keepon);
 winding_t *CopyWinding (const winding_t *w);
 
 void ClusterFlow (int clusternum);
-void BasePortalVis (void);
+void PortalBase (basethread_t *thread, portal_t *portal);
 void PortalFlow (threaddata_t *data, portal_t *portal);
 void CalcAmbientSounds (void);
 
diff --git a/tools/qfvis/source/base-vis.c b/tools/qfvis/source/base-vis.c
index 3470f1300..ca4d912e0 100644
--- a/tools/qfvis/source/base-vis.c
+++ b/tools/qfvis/source/base-vis.c
@@ -57,15 +57,12 @@
 #include "vis.h"
 #include "options.h"
 
-static int  clustersee;
-static set_t *portalsee;
-
 /*
 	This is a rough first-order aproximation that is used to trivially reject
 	some of the final calculations.
 */
 static void
-SimpleFlood (portal_t *srcportal, int clusternum)
+SimpleFlood (basethread_t *thread, portal_t *srcportal, int clusternum)
 {
     int			i;
     cluster_t  *cluster;
@@ -74,15 +71,15 @@ SimpleFlood (portal_t *srcportal, int clusternum)
 	if (set_is_member (srcportal->mightsee, clusternum))
 		return;
 	set_add (srcportal->mightsee, clusternum);
-    clustersee++;
+    thread->clustersee++;
 
     cluster = &clusters[clusternum];
 
     for (i = 0; i < cluster->numportals; i++) {
 		portal = cluster->portals[i];
-		if (!set_is_member (portalsee, portal - portals))
+		if (!set_is_member (thread->portalsee, portal - portals))
 			continue;
-		SimpleFlood (srcportal, portal->cluster);
+		SimpleFlood (thread, srcportal, portal->cluster);
     }
 }
 
@@ -99,86 +96,72 @@ test_sphere (sphere_t *sphere, plane_t *plane)
 }
 
 void
-BasePortalVis (void)
+PortalBase (basethread_t *thread, portal_t *portal)
 {
     int			i, j, k;
 	float		d;
-    portal_t   *tp, *portal;
+    portal_t   *tp;
     winding_t  *winding;
-	int         base_mightsee = 0;
 	int         tp_side, portal_side;
-	double      start, end;
 
-	start = Sys_DoubleTime ();
-	portalsee = set_new_size (numportals * 2);
-    for (i = 0, portal = portals; i < numportals * 2; i++, portal++) {
-		portal->mightsee = set_new_size (portalclusters);
+	i = portal - portals;
 
-		set_empty (portalsee);
+	for (j = 0, tp = portals; j < numportals * 2; j++, tp++) {
+		if (j == i)
+			continue;
 
-		for (j = 0, tp = portals; j < numportals * 2; j++, tp++) {
-			if (j == i)
-				continue;
+		// If the target portal is behind the portals's plane, then it
+		// can't possibly be seen by the portal.
+		//
+		// If the portal is in front of the target's plane, then the target
+		// is of no interest as it is facing counter to the flow of
+		// visibility.
 
-			// If the target portal is behind the portals's plane, then it
-			// can't possibly be seen by the portal.
-			//
-			// If the portal is in front of the target's plane, then the target
-			// is of no interest as it is facing counter to the flow of
-			// visibility.
-
-			// First check using the bounding spheres of the two portals.
-			tp_side = test_sphere (&tp->sphere, &portal->plane);
-			if (tp_side < 0) {
-				// The test portal definitely is entirely behind the portal's
-				// plane.
-				continue;	// entirely behind
-			}
-			portal_side = test_sphere (&portal->sphere, &tp->plane);
-			if (portal_side > 0) {
-				// The portal definitely is entirely in front of the test
-				// portal's plane.
-				continue;	// entirely in front
-			}
-
-			if (tp_side == 0) {
-				// The test portal's sphere touches the portal's plane, so
-				// do a more refined check.
-				winding = tp->winding;
-				for (k = 0; k < winding->numpoints; k++) {
-					d = DotProduct (winding->points[k],
-									portal->plane.normal) - portal->plane.dist;
-					if (d > ON_EPSILON)
-						break;
-				}
-				if (k == winding->numpoints)
-					continue;	// no points on front
-			}
-
-			if (portal_side == 0) {
-				// The portal's sphere touches the test portal's plane, so
-				// do a more refined check.
-				winding = portal->winding;
-				for (k = 0; k < winding->numpoints; k++) {
-					d = DotProduct (winding->points[k],
-									tp->plane.normal) - tp->plane.dist;
-					if (d < -ON_EPSILON)
-						break;
-				}
-				if (k == winding->numpoints)
-					continue;	// no points on front
-			}
-
-			set_add (portalsee, j);
+		// First check using the bounding spheres of the two portals.
+		tp_side = test_sphere (&tp->sphere, &portal->plane);
+		if (tp_side < 0) {
+			// The test portal definitely is entirely behind the portal's
+			// plane.
+			continue;	// entirely behind
+		}
+		portal_side = test_sphere (&portal->sphere, &tp->plane);
+		if (portal_side > 0) {
+			// The portal definitely is entirely in front of the test
+			// portal's plane.
+			continue;	// entirely in front
 		}
 
-		clustersee = 0;
-		SimpleFlood (portal, portal->cluster);
-		portal->nummightsee = clustersee;
-		base_mightsee += clustersee;
-    }
-	set_delete (portalsee);
-	end = Sys_DoubleTime ();
-	if (options.verbosity > 0)
-		printf ("base_mightsee: %d %gs\n", base_mightsee, end - start);
+		if (tp_side == 0) {
+			// The test portal's sphere touches the portal's plane, so
+			// do a more refined check.
+			winding = tp->winding;
+			for (k = 0; k < winding->numpoints; k++) {
+				d = DotProduct (winding->points[k],
+								portal->plane.normal) - portal->plane.dist;
+				if (d > ON_EPSILON)
+					break;
+			}
+			if (k == winding->numpoints)
+				continue;	// no points on front
+		}
+
+		if (portal_side == 0) {
+			// The portal's sphere touches the test portal's plane, so
+			// do a more refined check.
+			winding = portal->winding;
+			for (k = 0; k < winding->numpoints; k++) {
+				d = DotProduct (winding->points[k],
+								tp->plane.normal) - tp->plane.dist;
+				if (d < -ON_EPSILON)
+					break;
+			}
+			if (k == winding->numpoints)
+				continue;	// no points on front
+		}
+
+		set_add (thread->portalsee, j);
+	}
+
+	SimpleFlood (thread, portal, portal->cluster);
+	portal->nummightsee = thread->clustersee;
 }
diff --git a/tools/qfvis/source/qfvis.c b/tools/qfvis/source/qfvis.c
index 493e2920d..22b0b9c6e 100644
--- a/tools/qfvis/source/qfvis.c
+++ b/tools/qfvis/source/qfvis.c
@@ -74,6 +74,7 @@ bsp_t *bsp;
 options_t   options;
 
 static visstat_t stats;
+int         base_mightsee;
 
 int         portal_count;
 int         numportals;
@@ -408,6 +409,44 @@ LeafThread (void *_thread)
 	return NULL;
 }
 
+static void *
+BaseVisThread (void *_thread)
+{
+	portal_t   *portal;
+	int         thread = (int) (intptr_t) _thread;
+	basethread_t data;
+	set_pool_t  set_pool;
+	int         num_mightsee = 0;
+
+	memset (&data, 0, sizeof (data));
+	set_pool_init (&set_pool);
+	data.portalsee = set_new_size_r (&set_pool, numportals * 2);
+	do {
+		portal = GetNextPortal ();
+		if (!portal)
+			break;
+
+		if (working)
+			working[thread] = (int) (portal - portals);
+
+		portal->mightsee = set_new_size_r (&set_pool, portalclusters);
+		set_empty (data.portalsee);
+
+		PortalBase (&data, portal);
+		num_mightsee += data.clustersee;
+		data.clustersee = 0;
+	} while (1);
+
+	WRLOCK (stats_lock);
+	base_mightsee += num_mightsee;
+	UNLOCK (stats_lock);
+
+	printf ("thread %d done\n", thread);
+	if (working)
+		working[thread] = -1;
+	return NULL;
+}
+
 #ifdef USE_PTHREADS
 static void *
 WatchThread (void *_thread)
@@ -583,6 +622,20 @@ portalcmp (const void *_a, const void *_b)
 	return a->nummightsee - b->nummightsee;
 }
 
+static void
+BasePortalVis (void)
+{
+	double      start, end;
+
+	start = Sys_DoubleTime ();
+
+	RunThreads (BaseVisThread);
+
+	end = Sys_DoubleTime ();
+	if (options.verbosity > 0)
+		printf ("base_mightsee: %d %gs\n", base_mightsee, end - start);
+}
+
 static void
 CalcPortalVis (void)
 {