Non-Hodgkin lymphoma is associated with HIV infection. We investigated the epidemiology and aetiology of AIDS-related non-Hodgkin lymphoma by analysing data from cases reported to the Centers for Disease Control, Atlanta, USA, up to June 30, 1989. During this period 97,258 AIDS cases were reported, of whom 2824 (2.9%) had non-Hodgkin lymphoma. The condition was about 60 times more common in AIDS patients than in the general US population. 1686 cases were immunoblastic lymphoma, 548 primary lymphoma of the brain, and 590 Burkitt's lymphoma, a condition which is not normally associated with immunosuppression. The proportion of AIDS patients with immunoblastic lymphoma increased from 0% in those under 1 year old to 3.5% in those aged 50 or more. Primary lymphoma of the brain was constant at 0.6% for all ages. The frequency of Burkitt's lymphoma increased from zero in infants to a peak at 10-19 years of age (1.8%). Each type of lymphoma was twice as common in whites as in blacks and in men as in women. Lymphoma was most common in patients with haemophilia or clotting disorders and least common in those born in the Caribbean or Africa who had acquired HIV by heterosexual contact. Epidemiological data suggested that whilst infectious agents (eg, Epstein-Barr virus) may be associated with development of non-Hodgkin lymphomas in AIDS patients there was probably no single cause for all the types of lymphoma. Perhaps the most puzzling question is why Burkitt's lymphoma is commonly associated with HIV infection but not with other types of immunosuppression.