We have investigated plasma levels of endothelial cell products playing a role in hemostasis in 125 HIV-positive patients and 30 controls. Antigenic von Willebrand factor increased significantly with disease progression and was closely correlated with CD4+ cell counts and beta 2-microglobulin levels. Tissue-type plasminogen activator was normal in CDC II/III and CDC IVC2 patients and was slightly increased in AIDS patients, whereas plasminogen activator inhibitor was increased in each group, the stage of the disease not having any effect. Mean total protein S levels were lower in HIV-positive patients and, in 27.2% of the cases, were associated with a decrease in free protein S levels. Such abnormalities could be responsible for a hypercoagulable state in these patients and could be explained by endothelial cell damage during HIV infection. Whether this injury is due to HIV itself remains to be further investigated.