Previous studies utilizing enzyme histochemistry, electron microscopy, and immunohistochemistry have failed to establish the cell of origin in Kaposi's sarcoma. The authors have rigorously tested the prevailing hypothesis that the lesion defined as Kaposi's sarcoma is derived from vascular endothelial cells. They use seven markers to characterize endothelial cells: three antigens (Factor VIII-related antigen, HLA-DR/Ia, macrophage/endothelial antigens), three enzymes (5'-nucleotidase, ATPase, alkaline phosphatase), and lectin binding (Ulex europaeus I). They applied the markers first to normal skin and lymph node, and then to biopsy specimens from 40 patients with Kaposi's sarcoma. Normal blood vessel endothelium was positive for all seven markers, but normal lymphatic endothelium was negative for all of the markers except 5'-nucleotidase and Ulex europaeus lectin. The neoplastic cells in 40 cases of Kaposi's sarcoma closely resembled those of normal lymphatic endothelium but not those of blood vessel endothelium. This suggests that Kaposi's sarcoma may originate in lymphatic endothelium.