A total of 186 blood samples from 24 HIV-1 seropositive hemophiliac patients, monitored every four months for 29 months, were investigated for the presence of viral antigen in plasma. In addition, peripheral blood mononuclear cells (PBMC) were cultured for HIV-1, using normal PBMC as a target for replication. Antigenemia was detected in 51% of the patients and from PBMC in 87.5% of the patients. The incidence of HIV isolation in asymptomatic patients (42.8%) was similar to that found in symptomatic patients (51.4%). Patients with opportunistic infections had a higher incidence of lymphocytic viremia (p < 0.05). Plasma viremia was closely associated (p < 0.05) with low CD4+ counts and infection progression. The persistence of antigenemia was also a marker of a poor clinical course. In treated patients, plasma viremia was the marker that better correlated with the clinical course, and it did not appear during the first nine months of therapy. Zidovudine doses of > 500 mg/day significantly lowered the appearance of antigenemia and lymphocytic viremia (p < 0.05).