We studied a cohort of 299 HIV-positive individuals with known date of seroconversion to evaluate the role of Cytomegalovirus (CMV) in the natural history of HIV. The study population consisted of 236 initially CMV-positive patients, 55 CMV-negative subjects and 8 CMV seroconverters. The study endpoints were the decline to CD4+ < 200 x 10(6) cells/l, AIDS, and death. The cumulative risk of CMV disease and the survival after CMV disease were also investigated. At intake, there was no inter-group difference in sex, age, risk behaviours, history of hairy leucoplakia or herpes zoster and antiretroviral treatment. During the follow-up, 108 patients fell below 200 CD4+ x 10(6) cells/l, 72 developed AIDS and 63 died. Twenty-one subjects had CMV disease. The cumulative incidence of CMV disease in the cohort was 18.9%, and 23.3% within 8 and 9 years for the initially CMV-positive patients and 33.3% and 66.7% for the CMV seroconverters (log-rank test: p = 0.101). The median survival after CMV disease was 153 days (range: 28-855, interquartile range: 261), with a cumulative survival of 45.1%, 16.9% and 4.3% within 6, 12 and 18 months, respectively. On Cox's regression, the acute HIV seroconversion was an independent predictor of each endpoint, history of hairy leucoplakia or herpes zoster being associated only with CD4+ cell decline. Baseline CMV seropositivity was related to short survival (p = 0.037) and 2 x 2 inter-group comparison showed that older individuals with sexually acquired HIV who seroconverted to CMV had higher rates of progression to the study endpoints. Our data suggest that CMV infection influences the natural history of HIV disease and that CMV disease strongly affects the survival of the HIV-positive patients.