Studies were performed to define the immunologic status of various groups of homosexual men including homosexual men with Kaposi's sarcoma, healthy homosexual men who were of similar ages to the homosexual patients with Kaposi's sarcoma and homosexual men with hyperplastic lymphadenopathy. Heterosexual men with Kaposi's sarcoma were also studied. Immunologic parameters which were examined included serum immunoglobulin levels, enumeration of B cells, T cells, and T-cell subsets, and quantitatlon of lymphocyte responsiveness to phytohemagglutinin (PHA) and pokeweed mitogen (PWM). Significant immunologic abnormalities were observed in all three groups of homosexuals studied. These were most severe in the homosexuals with Kaposi's sarcoma, somewhat less severe in homosexual men with lymphadenopathy, and least marked but still significant in healthy homosexual men. Heterosexual men with Kaposi's sarcoma displayed essentially normal immunologic profiles. The possible etiologic factors underlying the immunologic abnormalities In the male homosexual population studied and the role of an altered immune system in the development of and the fulminant course of Kaposi's sarcoma in these patients are discussed.