We examined 26 patients with human immunodeficiency virus-1 (HIV-1)- associated Kaposi′s sarcoma (KS), and 76 HIV-1-infected (HIV-1+) people without KS or uninfected (HIV-1-) controls for the presence of circulating KS-like spindle cells. Adherent cells that had spindle morphology and several characteristics of spindle cells of KS lesions (KS cells) were identified in the peripheral blood mononuclear cell fraction only after culture in the presence of conditioned medium (CM) from activated lymphocytes. The peripheral blood-derived spindle cells (PBsc) expressed a variety of endothelial cell markers, such as Ulex europaeus I lectin, EN4, EN2/3, EN7/44, CD13, CD34, CD36, CD54, ELAM-1, and HLA-DR. However, they were negative for CD2, CD19, PaIE, and factor VIII-related antigen. The PBsc produced angiogenic factors as evidenced by the ability of CM from these cells to promote growth of normal vascular endothelial cells. In addition, subcutaneously injected PBsc stimulated angiogenesis in vivo in athymic nude mice. We determined that the number of PBsc grown from the peripheral blood of HIV-1+ patients with KS or at high risk to develop KS were increased by 78- fold (P = .0001) and 18-fold (P = .005), respectively, when compared with HIV-1- controls. The number of spindle cells cultured from the HIV- 1+ patients at low risk for developing KS, eg, HIV-1+ injection drug users, showed no statistical increase when compared with HIV-1- controls. The presence of increased PBsc with characteristics of KS cells in HIV-1+ KS patients or patients at high risk for developing KS gives insights into the origin of KS cells and may explain the multifocal nature of the disease. In addition, this may be useful in predicting the risk of KS development.