Kaposi's sarcoma is a multifocal neoplasm of the reticuloendothelial system which has generated great interest recently because of its association with immunosuppressive therapy and its occurrence within a group of homosexual men. Our case demonstrates a relationship between the discontinuation of immunosuppressive therapy and tumor regression in a renal transplant recipient. The clustering of cases in geographic areas and within ethnic groups implies that both heriditary and environmental factors contribute to the pathogenesis of this tumor. Recently described cases have occurred in a setting of multiple infections and concurrent impairment of cell mediated immunity. This suggests that Kaposi's sarcoma may be a model of chronic antigen stimulation leading to decreased immune responsiveness and subsequent unchecked growth of transformed cells.