The incidence of neutralizing isoantibody formation to infused factor VIII in a cohort of 67 hemophilia A patients, born between January 1, 1971 and April 30, 1990, who had been treated exclusively with lyophilized cryoprecipitate, was 6% (5.3 per 1,000 patient years of observation). The age-dependent cumulative risk was 4.6% at 4 years of age and 6.7% at 8 years of age. Recent reports in patients treated with a variety of more pure concentrates show a much higher incidence of inhibitor formation and tend to be used as a reference when new concentrates are introduced. We believe that a patient group, such as the one studied here, is a more suitable reference population because these patients have been exclusively treated with a single factor VIII preparation.