Patients with severe hemophilia A often develop inhibitors (antibodies) against transfused factor VIII. One hundred thirteen Polish patients with severe hemophilia A, who had been treated on demand with cryoprecipitate until 1992 and exclusively with factor VIII concentrates after 1995, were examined for intron 22 inversion by Southern blotting and the presence and magnitude of inhibitor activity in blood as determined by the Bethesda assay. The patients' ages ranged 4?67 years (mean: 33.7?12.4 years, median: 32 years). The number of patients with the inversion amounted to 57, while in 56 patients the mutation types were unknown; 47 patients had a distal and 10 patients a proximal type of inversion. Thirteen patients with inversions (22.8%) were found to have inhibitor in their blood. Most patients (14 out of 15) who developed inhibitors in the course of cryoprecipitate therapy were high responders. Conversely, 4 of 5 patients treated between 1992 and 1995 with both cryoprecipitate and intermediate-purity factor VIII concentrates were low responders. One multitransfused patient who had remained inhibitor-free on cryoprecipitate therapy developed inhibitor after receiving a large dose of factor VIII concentrate during surgery. None of these 5 patients developed inhibitors during their 12?40 years of treatment with cryoprecipitate, suggesting that it was less immunogenic than factor VIII concentrates. The prevalence of the intron 22 inversion mutation of the factor VIII gene in Polish hemophiliacs is similar to that in other European countries. Treatment regimens with either cryoprecipitate or virus-inactivated plasma-derived Factor VIII concentrates may affect inhibitor formation in hemophilia A patients.