BACKGROUND: CD4+ T cells restrict parasitaemia during the first attack of falciparum malaria; humoral immunity, develops weeks later and protects against reinfection. HIV infection may affect severity of falciparum malaria and development of protective immunity. AIMS: To study the prevalence of HIV infection in Indian patients with severe falciparum malaria and its effect on severity of illness and recurrences of and mortality related to malarial infection. PATIENTS: Consecutive patients with severe falciparum malaria and voluntary blood donors. SETTING AND DESIGN: Prospective cohort study in a university hospital in Mumbai. RESULTS: Five (11.6%) of 43 patients and 521 (1.8%) of 28749 blood donors had HIV infection (OR 7.1, 95% CI = 2.8 to 18.2, p=0.001). Clinical features, APACHE II score, number of organs affected, parasite index and mortality in patients with and without HIV infection were comparable. CD4+ counts were < 500 cells/ microl in 2 patients and normal in 3. Opportunistic infections including pulmonary tuberculosis in one patient (CD4+ counts > 500 cells/ microl), and oral candidiasis in two (CD4+ counts 275 and 250 cells/ microl) were noted. One patient developed fatal Pneumocystis carinii pneumonia two weeks after recovering from malaria. P. falciparum infection recurred in 2 of the 4 HIV infected survivors and in none of 31 survivors without HIV infection (RR 38.8, 95% CI 2.2 to 671, p=0.01). CONCLUSIONS: HIV infection is associated with increased risk of severe malaria even with normal CD4+ counts; severity of disease and mortality are not increased. However, prior HIV infection impairs protective immune response to Plasmodium falciparum in residents of hypoendemic areas.