diff --git a/lemon/preflow.h b/lemon/preflow.h
--- a/lemon/preflow.h
+++ b/lemon/preflow.h
@@ -576,12 +576,18 @@
     void startFirstPhase() {
       _phase = true;
 
-      Node n = _level->highestActive();
-      int level = _level->highestActiveLevel();
-      while (n != INVALID) {
+      while (true) {
         int num = _node_num;
 
-        while (num > 0 && n != INVALID) {
+        Node n = INVALID;
+        int level = -1;
+
+        while (num > 0) {
+          n = _level->highestActive();
+          if (n == INVALID) goto first_phase_done;
+          level = _level->highestActiveLevel();
+          --num;
+          
           Value excess = (*_excess)[n];
           int new_level = _level->maxLevel();
 
@@ -647,14 +653,22 @@
           } else {
             _level->deactivate(n);
           }
-
-          n = _level->highestActive();
-          level = _level->highestActiveLevel();
-          --num;
         }
 
         num = _node_num * 20;
-        while (num > 0 && n != INVALID) {
+        while (num > 0) {
+          while (level >= 0 && _level->activeFree(level)) {
+            --level;
+          }
+          if (level == -1) {
+            n = _level->highestActive();
+            level = _level->highestActiveLevel();
+            if (n == INVALID) goto first_phase_done;
+          } else {
+            n = _level->activeOn(level);
+          }
+          --num;
+
           Value excess = (*_excess)[n];
           int new_level = _level->maxLevel();
 
@@ -720,19 +734,9 @@
           } else {
             _level->deactivate(n);
           }
-
-          while (level >= 0 && _level->activeFree(level)) {
-            --level;
-          }
-          if (level == -1) {
-            n = _level->highestActive();
-            level = _level->highestActiveLevel();
-          } else {
-            n = _level->activeOn(level);
-          }
-          --num;
         }
       }
+    first_phase_done:;
     }
 
     /// \brief Starts the second phase of the preflow algorithm.