We conducted a retrospective study on the rate of occurrence of inhibitor (factor VIII or factor IX antibody) formation in Japanese hemophiliacs, and examined some possible causes. The records of 51 medical institutions were analyzed. Inhibitors were found in 6.08% (101/1661) of the patients with hemophilia A and 4.17% (13/312) of those with hemophilia B. The median age for antibody formation was 11 (0.8-75.9 years old, n = 94) with hemophilia A, and 4 (1-42 years old, n = 13) with hemophilia B. Among the patients with inhibitors, 69.7% (62/89) of the hemophilia A cases and 81.8% (9/13) of the hemophilia B cases were classified as high responders (> 10 Bethesda units (BU)/ml). The median time from initial exposure to plasma factor concentrates until inhibitor formation (exposure days) was 46 days (4-162 days, n = 51) with hemophilia A and 30 days (2-151 days, n = 8) with hemophilia B. Studies on 15 families with two hemophiliacs showed a non-random distribution of antibodies. Gene analysis was performed in three patients with hemophilia A. A large gene deletion was detected in two cases and a nonsense mutation in one. Analysis of eight patients with hemophilia B revealed a large deletion in five cases and a nonsense mutation in three. The antibodies in these eight hemophiliacs whose mutation site was identified were found between the ages of one and nine. Of these, six patients were less than 4 years old. All these patients were considered high responders as they all exhibited a high inhibitor potency. Of the six hemophiliacs for whom the number of exposures could be determined, four became high responders with fewer than 10 exposures.