After several hours of ischemia an incomplete return of blood flow has been reported in brain, kidney, skeletal muscle, and heart. The mechanisms responsible for the no-reflow phenomenon have been unclear, and perivascular edema, platelet or red cell plugs, and interstitial hemorrhage have been implicated. In the present study evidence is provided that leukocyte entrapment in capillaries might contribute to no reflow. Leukocytes are large and stiff cells, which adhere to vascular endothelium naturally and are known to alter in their adherence properties under a variety of conditions. Accordingly, 11 open-chest dogs were studied, 1-5 hours after left anterior descending coronary artery occlusion. Reperfusion (9 dogs) at 75 mm Hg arterial pressure was accomplished with Ringer's lactate and carbon suspension as a marker for capillary patency. In non-ischemic tissue, 98% of the capillaries contained carbon, rare leukocytes, and few erythrocytes, whereas tissue from the distribution of the occluded artery was heterogeneous: 60% of the capillaries had no carbon, high hematocrits, and approximately one leukocyte per unbranched capillary; 40% demonstrated reflow and no leukocytes. A significant correlation between capillaries without carbon (no reflow) and the frequency of leukocytes remaining in these capillaries indicated that leukocytes were present in obstructed capillaries. Furthermore, the frequency of leukocytes remaining after the washout with lactate was ten times greater than in normal arrested heart muscle without washout. Our results suggest that progressive leukocyte capillary plugging during myocardial ischemia contributes to preventing full restoration of capillary flow upon reperfusion.