Recent data published on the prevalence of inhibitors to factor VIII in hemophiliacs on treatment show great variations, with prevalence rates ranging from 3.6 to 14.2%. We have studied the cumulative risk of inhibitor development in a cohort of 62 patients with hemophilia A. All patients were born after 1960, were natives of the Vienna area, had a factor VIII activity of less than 5%, and were treated at least once. Using the method of Cutler and Ederer, the cumulative risk of inhibitor development was found to be 24% at the age of 25 years. Most inhibitors developed between the ages of 3 and 7 years. The current prevalence of F VIII inhibitors in the group of patients studied is 17.5%. It is concluded that prevalence data underestimate the true risk of inhibitor development.