The presence of the antibody to human immunodeficiency virus (HIV) was investigated in 487 serum samples withdrawn from 216 haemophiliacs (186 haemophiliacs A and 30 B) who came to the Pisa Centre between 1977 and 1986. Results show that a considerable proportion of haemophiliacs (15%) were already positive in 1982. In haemophiliacs A, who were treated up to 1983 with concentrates made from US plasma, but from 1984 on began to use concentrates made from Italian plasma, this proportion in 1986 was 16%. In haemophiliacs B who have continued using concentrates from US plasma, the prevalcence rate of Anti-HIV was found to be much higher (63%). As regards the contact with the virusof hepatitis B (HBV), antibodies werefound in 64% of haemophiliacs and theviral antigens in 6%. 137 of these haemophiliacs (112 A and 25 B) were examined between 1985 and 1986 for indexof hepatic cytoli-sis, immunological and clinical status. In 65% of haemophiliacs the level of serum glutamic pyruvate transaminase (SGPT) was mildly increased, perhaps for the presenceof chronic hepatitis. In Anti-HIV negative subjects we found a decrease of T helper/T suppressor (Th/Ts) ratio, with a mean of 1.3 (in controls, 1.7),due especially to an increase of Ts (mean 0.8 × 103Vcu.mm.; incontrols, mean 0.6). Also in Anti-HIVpositive haemophiliacs there was a decrease ofTh/Ts ratio (mean 1.1), but this was mainly due to a decrease of Th (mean 0.7 × 103/cu.mm.;in controls, mean 1.1). Clinical evaluation of the Anti-HIVpositive subjectsshowed 2 patients with AIDS related illness with opportunistic infections and 13 haemophiliacs (33% of seropositive subjects) withone or more of these abnormalities: thrombocytopenia, lymphoadenopathy, slight persistentfever, diarrhea.