The biopsies of all lesions clinically thought to be suspicious for Kaposi's sarcoma (KS) were reviewed over a 15-month period. A diagnosis of KS was made in 40 of 106 biopsies (38%). The cases in which a diagnosis other than KS was made included dermatofibroma, hemangioma, and scar. This second group comprised 59 of 106 cases (56%). A third group included some lesions that had an atypical vascular proliferation, but in which the changes were insufficient for a definite diagnosis of KS. The presence of abnormally shaped vessels, especially those classified as irregular, was the best single criterion to diagnose KS in its early stages. In later stages, the neoplasm assumes a nodular configuration with typical, slitlike vascular channels. At the periphery of such nodules dilated, irregularly shaped vessels similar to those of the early lesions are often seen. The histologic features which help in the diagnosis of KS from other histologic entities are reviewed.