A group of 173 subjects affected by congenital clotting factor deficiencies was evaluated with regard to the impact of HIV infection. On the whole, 78 patients (45%) were found to be HIV Ab-positive. As of March, 1988, of the seropositive patients, 63 (80.8%) had an asymptomatic HIV infection (Group II/CDC), three (3.8%) had a persistent generalized lymphadenopathy (Group III/CDC). The 12 (15.4%) remaining patients could be classified in Group IV of the CDC classification due to their symptoms and signs; in particular, 10 came under surveillance case definition for AIDS. Assay for the HIV antigen was positive in 14 (17.9%) seropositive hemophiliacs. With regard to the immunological features, our data clearly show that a sharp decline in the number of CD4+ cells was associated with symptomatic forms of the disease. An evaluation of the time elapsed from seroconversion to the appearance of the symptomatic clinical condition showed an average incubation of 37 months.