To determine whether leukocytes play an important role in the pathogenesis of the vascular injury (increased vascular permeability and resistance) associated with ischemia-reperfusion, isolated canine gracilis muscles were perfused with autologous whole blood or with whole blood that had been depleted of leukocytes (primarily granulocytes) using Leukopak filters. The osmotic reflection coefficient for total plasma proteins, isogravimetric capillary pressure, and total vascular resistance was determined for the following conditions: control, ischemia (4 h inflow occlusion) plus reperfusion with whole blood, and ischemia plus reperfusion with granulocyte-depleted whole blood. Reperfusion with whole blood was associated with a reduction in the osmotic reflection coefficient from 0.96 to 0.61, whereas isogravimetric capillary pressure was reduced by 40%, indicating a dramatic increase in vascular permeability. Total vascular resistance was increased approximately twofold. Reperfusion with leukocyte-depleted blood largely prevented the increases in vascular permeability and resistance. These data suggest that leukocytes play a major role in the pathogenesis of ischemia-reperfusion injury in skeletal muscle.