The epidemiology of HIV associated non-Hodgkin's lymphoma (NHL) was investigated in 6550 European patients with AIDS. NHL was diagnosed in 3.5% of all patients at the time of the AIDS diagnosis. Although the probability of being diagnosed with NHL at AIDS diagnosis was significantly higher among intravenous drug users than among homosexual men, and was associated with increasing age, the observed incidences of NHL were more strikingly similar than any differences. The rate of developing NHL after a previous AIDS diagnosis was 2.4 per 100 patient years of follow-up, and remained constant during a 5-year follow-up period. While primary brain lymphomas comprised only 9% of NHL diagnosed at the time of AIDS, they comprised 38% of NHL diagnosed after AIDS (p < 0.001). The prognosis for patients with NHL at AIDS diagnosis was poor with a median survival of 5 months. A diagnosis of primary brain lymphoma was uniformly associated with a poor outcome. It is concluded that the probability of developing NHL in late stage HIV infection is lower than previously anticipated from the results of small studies on patients receiving long-term anti-retroviral therapy.