Enothelial injury is assumed to be of pathogenetic significance in the development of graft stenoses, which remain a major cause of failure of peripheral bypasses. The aim of this study was to assess endothelial injury related to infrainguinal bypass surgery by indium-111 platelet scintigraphy. In 28 patients undergoing in situ vein (n = 24), composite vein-polytetrafluoroethylene (PTFE) (n = 1) or PTFE (n = 3) bypass surgery, assumed vascular injuries were recorded intraoperatively. Autologous indium-111-labelled platelets were injected into the inflow artery immediately after restoration of flow in the graft. Platelet deposition was assessed by gamma-camera images of thigh and crus obtained 4 and/or 24 h after surgery. Areas of focally increased activity were recorded and graded as moderate or intense. In the 24 vein bypasses, a median of two (range 0-5) areas of focally increased radioactivity were seen at the proximal anastomosis (n = 21), in the body of the graft (n = 20) or at the distal anastomosis (n = 9). The activity was moderate in 27 cases and intense in 23 cases. Scintigraphic evidence of focal platelet aggregation in vein grafts was not correlated with preoperative antiplatelet therapy or vein graft diameter. Only 2 of the 20 intragraft platelet depositions occurred in areas where intra-operative vascular injury was suspected. In the composite graft and the PTFE grafts, diffuse activity was observed throughout the entire bypass. In conclusion, focal activity accumulations, suggesting localized endothelial injury, were observed in the majority of in situ vein bypasses, in particular at the sites of the anastomoses. Prosthetic bypasses were characterized by diffuse platelet aggregation.