The origin of spindle-shaped cells in Kaposi's sarcoma (KS) remains controversial. Non-specific histochemical reactions, electron microscopic examinations and immunostainings using antibody against factor VIII-related antigen (F VIII-RAG) and Ulex europaeus agglutinin I (UEAI) lectin as endothelial markers have given contradictory results. Immunohistochemical techniques were applied to 7 frozen skin biopsy specimen of KS from 5 elderly Mediterranean people and 1 renal allograft recipient, and a group of 27 other frozen cutaneous tumours including haemangio and lymphangiosarcomas, benign vascular lesions and various epithelial, melanocytic, fibrohistiocytic, fibrosarcomatous and muscular tumours. Using UEAI and antibodies against F VIII-RAG, HLA-DR and vimentin, a large proportion of positive KS spindle cells was found in all cases whereas cells were negative for keratin. Among the various immunoreactivity patterns observed in this study, a unique immunohistochemical profile was demonstrated for KS, angiosarcoma and endothelial cell, which strongly supports the endothelial origin of spindle cells in KS. Whereas F VIII-RAG, HLA-DR, vimentin and UEAI were sensitive endothelial markers, only F VIII-RAG appeared specific for endothelial cells since UEAI stained 2 squamous cell carcinomas and HLA-DR and vimentin were present in various mesenchymal and melanocytic tumours.