Recent studies suggest that polymorphonuclear leukocytes (PMNs) may cause additional myocyte injury during reperfusion of ischemic myocardium. The present study was done to investigate whether PMNs accumulate in myocardium during early reperfusion after reversible or irreversible ischemic injury. Open-chest anesthetized dogs underwent circumflex coronary occlusions for 12 min (n = 5), 40 min (n = 8), or 90 min (n = 8), followed by 1 h of reperfusion. Autologous PMNs were radiolabeled with 111In and reinjected to quantitate myocardial PMN influx during reflow. 125I-labeled albumin was injected simultaneously to correct for 111In associated with plasma proteins in myocardial tissue. The number of PMNs was determined in the inner, middle, and outer one-third of nonischemic and ischemic-reperfused myocardium. In the 12-min group, 40% fewer PMNs were present in the reperfused than in the nonischemic control tissue. In contrast, in both the 40- and 90-min groups, PMN accumulation was two- to sixfold greater in the ischemic-reperfused than nonischemic myocardium, with a transmural gradient of PMN influx increasing from the outer to inner layers. Collateral blood flow, measured with radioactive microspheres, was not significantly different among the three groups. The failure of PMNs to accumulate during reperfusion after 12 min of ischemia does not support the hypothesis that PMNs contribute to postischemic dysfunction of reversibly injured myocytes. Whether PMNs caused cell death during early reperfusion after longer ischemic episodes remains unknown; however, the rapidity of PMN accumulation in the zones of predicted infarction is consistent with this possibility.