We describe the histologic and clinical features of non-Hodgkin's lymphoma diagnosed between January 1980 and December 1983 in 90 homosexual men from San Francisco, Los Angeles, Houston, and New York. The median age was 37 years, with an age distribution identical to that for cases of AIDS reported to the Centers for Disease Control. Sixty-two per cent of the patients had high-grade (aggressive) subtypes of lymphoma, 29 per cent had subtypes of intermediate grade, and 7 per cent had low-grade subtypes. Histologic subtypes and malignant cell phenotypes were consistent with a B-cell origin. All but two men had extranodal lymphoma: central-nervous-system, bone-marrow, bowel, and mucocutaneous sites were most commonly involved. Thirty-five of 66 evaluable men (53 per cent) had complete responses to combination chemotherapy or radiotherapy or both, and thus far, 19 (54 per cent) of them have had a relapse. Mortality and morbidity were closely related to prodromal manifestations; death or illness have occurred in 19 (91 per cent) of the 21 men who presented with AIDS, in 26 (79 per cent) of the 33 who presented with generalized lymphadenopathy, and in 5 (42 per cent) of the 12 who had no prodromal manifestations. Mortality rates analyzed according to histologic grade were higher than currently reported rates in other patient populations. Kaposi's sarcoma or severe opportunistic infections characteristic of AIDS developed in 14 of 33 men (42 per cent) who presented with generalized lymphadenopathy and in 3 of 12 (33 per cent) without prodromal manifestations. We conclude that non-Hodgkin's lymphoma in members of an AIDS risk group is a serious manifestation of AIDS and the AIDS-related complex.