The incidence of non-Hodgkin's malignant lymphoma is known to be increased in patients who are seropositive for the human immunodeficiency virus (HIV). We report here a multicentre retrospective study of 21 HIV-positive patients with non-Hodgkin's lymphoma seen between 1985 and 1987. All phenotype B lymphomas of intermediate or high malignancy grade according to the Working Formulation are difficult to classify histologically. Because of this problem, reexamination of the specimens by several pathologists and perhaps also the use of other morphological prognostic criteria, such as mitotic index, seem to be desirable. In more than one-third of our patients the presence of a lymphoma led to the finding of HIV seropositivity in subjects who were all issued from populations at risk. Median age was 39 years. Spread evaluation showed stage III or IV in most cases with, in 4 out of 5 patients, extranodal sites, notably the neuromeningeal system, liver, gastrointestinal tract and bone marrow. The median overall survival was 5 months, but in April 1989 2 patients had survived for more than 30 months. Obtaining complete remission (11/21 cases) was imperative for a 10 months' survival. Eight of the 11 patients in whom complete remission was obtained had received the heavy induction chemotherapy required by the degree of malignancy, but no death due to drug toxicity was recorded. 17 patients died, with active lymphoma (12 cases) and/or infection (8 cases) being documented at the time of death. The finding of more than 500/sq. mm CD4 lymphocytes in peripheral blood in 10 cases while the lymphoma was developing, and the heterogeneity of the Epstein-Barr virus (EBV) profile in serum raise the question of the role played by T-cell immunodeficiency and by EBV infection in the physiopathogenesis of these lymphomas.