To investigate the interval between HIV-1 infection and the development of clinical AIDS among Italian patients with congenital coagulation disorders, a national cohort study was undertaken in 1988. Information was collected both retrospectively and prospectively on 499 HIV-1-positive patients enrolled in an ongoing national registry of patients with congenital coagulation disorders. Two methods were used to estimate each patient's seroconversion date: the mid-point between the last negative (either known or estimated) and the first positive test, and the median under a Weibull distribution, which was assumed to fit seroconversion data. The two methods of estimating the seroconversion time yielded similar results. The actuarial incidence of AIDS was estimated using the Kaplan-Meier survival analysis at 12.8% (95% confidence interval = 9.7-15.9) over 7 years for Italian haemophiliacs. Progression appears to be slow in the first 5 years after the infection, and to rise steadily thereafter. A strong association between faster progression and older age at seroconversion was found. Zidovudine-treated individuals seem to have a slower progression than untreated individuals, after controlling for CD4, but there was no association between progression and type and severity of the congenital disorder