BACKGROUND: Although AIDS is less frequent following HIV-2 than HIV-1 infection, it is unclear whether the clinical picture and clinical course of AIDS are similar in the two infections. OBJECTIVES: To compare the pattern of AIDS-defining events, CD4 cell count at the time of AIDS diagnosis, survival from time of AIDS, and CD4 cell count near time of death in HIV-1 and HIV-2-infected patients. METHODS: Adult patients with AIDS who attended the clinics of the MRC in The Gambia were enrolled. AIDS was diagnosed according to the expanded World Health Organization case definition for AIDS surveillance (1994). RESULTS: Three hundred and forty-one AIDS patients with HIV-1 and 87 with HIV-2 infection were enrolled. The most common AIDS-defining events in both infections were the wasting syndrome and pulmonary tuberculosis. The median CD4 cell count at AIDS was 109 cells/microl in HIV-1 and 176 in HIV-2 (P = 0.01) and remained significantly higher in HIV-2 after adjustment for age and sex (P = 0.03). The median time to death was 6.3 months in HIV-1 and 12.6 months in HIV-2-infected patients (P = 0.03). In a multivariable analysis adjusting for age, sex and CD4 cell count, the mortality rates of HIV-1 and HIV-2-infected patients were similar (P = 0.25). The median CD4 cell count near time of death was 62 and 120 cells/microl in HIV-1 and HIV-2-infected patients, respectively (P = 0.02). CONCLUSIONS: HIV-2 patients have a higher CD4 cell count at the time of AIDS, and a longer survival after AIDS. The mortality after an AIDS diagnosis is more influenced by CD4 cell count than HIV type.