Use of high purity and recombinant factor VIII (FVIII) concentrates has been thought to be associated with an increased incidence of FVIII inhibitors in patients with severe haemophilia A. Comparison with comparable historical control groups has suggested that the true incidence of inhibitors in patients with severe haemophilia A was approximately 20-25%, similar to the incidence seen with new high purity and recombinant FVIII products. We have conducted a study of inhibitor development in a cohort of 37 boys with severe haemophilia A (VIII:C <2 u/dl) exposed only to a single FVIII concentrate (BPL 8Y) with no previous blood or blood product exposure. This factor VIII concentrate is an intermediate purity product with a specific activity of approximately 2 IU/mg protein and contains well preserved von Willebrand factor multimers. It is manufactured by conventional fractionation technologies and terminally dry heat treated at 80 degrees C for 72 h.