The clinically uninvolved skin of 4 patients with well-developed AIDS was investigated by electron microscopy. All biopsy specimens had vascular abnormalities: protruding endothelial cells, vascular channels reduced to slits, gaps within the vascular walls, and extravasated erythrocytes. These features are similar to those described in early lesions of Kaposi's sarcoma. These findings suggest that blood vessels of the clinically uninvolved skin of AIDS patients are potential sites of Kaposi's sarcoma lesions.