Serum specimens from 252 recipients of factor VIII and/or factor IX (237 hemophiliacs and 15 nonhemophiliacs), 99 blood transfusion recipients, 269 chronic diseases, and 366 healthy subjects (included blood donors, hospital personnel and family members of hemophiliacs) were tested for reactivity to lymphadenopathy-associated virus/T-lymphotropic retrovirus type III (LAV/HTLV-III) by ELISA test kit (Abbott), and the presence of the antibodies to LAV/HTLV-III was confirmed by indirect immunofluorescence assay (IFA). Seropositivity rates were 38.4% (91 of 237) in hemophiliacs, 13.3% (2 of 15) in nonhemophiliac recipients of factor VIII and/or factor IX, and 8% (2 of 27) in male homosexuals. None of the other patients and healthy individuals had antibodies to LAV/HTLV-III. In serological follow-up study on 40 seronegative and 10 seropositive hemophiliacs, 4 of the seronegative hemophiliacs converted to seropositive, and 7 of seropositive hemophiliacs had the antibody titers elevated by 2 to 4 fold within the period for 3.5 to 13 months after first blood samplings. Comparative analysis of the absolute numbers of T helper (T4) and T suppressor (T8) lymphocytes between the seropositive and seronegative hemophiliacs revealed that more than 50% of both seropositive and seronegative hemophiliacs had lower T4 lymphocyte numbers and higher T8 lymphocyte numbers than those of normal subjects. Particularly, over 80% of seropositive hemophiliacs had lower T4 lymphocyte numbers. Two Japanese AIDS cases (a hemophiliac B and a male homosexual) were found in the present study. Both died of pneumonia, and were confirmed officially as AIDS cases.