We report malignant lymphoma in 27 homosexual men, of whom 22 had high-grade lymphomas (B-cell immunoblastic sarcoma or small non-cleaved lymphoma) and five had low-grade disease. Antibody to human T-cell lymphotropic virus type III (HTLV-III) was present in 13 (87%) of 15 with high-grade lymphoma and in two (40%) of five with low-grade disease. In contrast, only one (9%) of 11 "control" heterosexual patients with high-grade lymphoma had antibody to HTLV-III, while such antibody was found in none of 40 asymptomatic heterosexual controls and in 17 (55%) of 31 asymptomatic homosexual men. Of the homosexual lymphoma patients, 85% presented with disease in extranodal sites, including the central nervous system and rectum, and 81% had reversed T-helper/suppressor ratios. Median survival, despite treatment, is eight months. The acquired immunodeficiency syndrome-related lymphomas in homosexual men are extranodal, high-grade, B-lymphoid tumors, associated with exposure to HTLV-III and unusual clinical characteristics.