The effects of sera and of platelet-poor plasma from patients with scleroderma on endothelial cell survival in vitro were studied. The survival ratio of rat heart endothelial cells was studied both in 10% test serum and in 10% platelet-poor plasma. Sera from patients with scleroderma decreased the survival ratio significantly when compared with sera from normal controls. In contrast, there was no significant difference between platelet-poor plasma from patients with scleroderma and that from normal controls. Our data indicate that platelets in the patients with scleroderma may cause vascular damage by affecting endothelial cell survival.