Serious infections, neoplasms, and immunologic abnormalities have been found in homosexual men. We describe the development of malignant lymphoma in six such patients, three of whom had persistent, generalized lymphadenopathy. In biopsies done before the lymphoma developed, the lymphadenopathy was characterized morphologically by a distinctive pattern of B-cell follicular hyperplasia. All lymphomas were of B-lymphocytic origin, including B-cell immunoblastic sarcoma; small noncleaved, Burkitt-like lymphoma; and plasmacytoid lymphocytic lymphoma. Extranodal presentation with B symptoms occurred in five patients. Median age of our patients was 33 years. Three patients had histories of repeated systemic infections. The peripheral blood lymphocyte count was depressed in four, with depression of OKT 4+ (helper phenotype) cell levels and reversal of the T-helper:T-suppressor ratio in all. We conclude that these patients are at risk for the development of abnormalities of the B-lymphocytic system, manifested by abnormal hyper-B-cell response in enlarged reactive lymph nodes and aggressive, extranodal B-cell lymphomas.