Interval gangrene of the lower extremity: a complication of vascular surgery. Interval gangrene, segmental ischemic necrosis proximal to a functioning distal anastomosis, is a rare complication after successful peripheral vascular reconstruction. Previous reports have demonstrated the gravity of this event in that major limb amputation was required in all cases. Two cases are presented to emphasize the need for maintaining segmental collateral circulation after successful distal extremity bypass. Despite a satisfactory result after distal (inframalleolar) bypass of a popliteal aneurysm, one patient had progressive ischemic gangrene of the upper leg and eventually required amputation. The contralateral limb was successfully managed by distal ligation of the superficial femoral artery, which maintained collateral flow from the proximal superficial femoral artery to the knee and leg. The second patient required a microvascular free flap to replace tissue loss and provide vascular graft coverage after initial multiple bypass failures and a final successful remote reconstruction to the dorsal artery of the foot that excluded the popliteal-crural collateral network. Patients with inadequate collateral circulation or disruption of the profundus or geniculate collateral pathways may require revascularization of sequential vascular beds. Recognition of the potential for interval gangrene is essential since the likelihood of its occurrence will increase in proportion to the number of distal bypasses being performed for limb salvage. Although adjunctive procedures will not completely eliminate the possibility of interval gangrene, awareness of this phenomenon with attention to the segmental collateral circulation can decrease the incidence of its occurrence. 