In 58 hemophilia A patients aged 1-18 years (mean 9.5 +/- 4.7 years), the prevalence of inhibitors was found to be 27% by the Bethesda method in November 1995. Inhibitor activity was not detected in any of 14 patients with mild hemophilia while it was present in 9 of 27 (33%) patients with moderate, and 7 of 17 (41%) with severe disease. During follow-up, the inhibitors were transient in 10 of 16 patients (17%) and the prevalence of inhibitors was 10% at the end of the study. Our study has demonstrated that the patients' age, factor VIII (F VIII) coagulant activity levels, type of F VIII replacement therapy, and frequency of F VIII administration affect inhibitor development, and these factors should be considered in the follow-up of hemophiliacs.