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The experience with immune tolerance (IT) induction therapy for haemophilia patients with inhibitors, instituted during 1990-97 at 17 haemophilia treatment centres in the western United States, was reviewed. IT was instituted in 104 of 139 (75%) of all identified haemophilia A and haemophilia B patients with inhibitors. Doses and schedules for IT varied but most patients were treated with a daily administration of 25-200 units kg-1 day-1. Successful immune tolerance (defined as Bethesda Unit titres < 1.0) was achieved in 57 of 81 (78%) patients who completed therapy. IT success was significantly greater for those patients with historic titres < 100 BU (P < 0.0003) and those with titres of < 10 BU at initiation of IT (P < 0.0001). IT success was uniformly achieved in 16 patients with low to moderate responding inhibitors. IT in infants < 2 years of age was problematic due to the presence of high titre inhibitors and complications of venous access. Frequent complications of IT included increased bleeding during intravenous access device insertion (20%) and infections associated with venous access devices (64%). IT is widely accepted as a treatment modality for haemophilia patients with inhibitors. Patient participation in IT regimens and management during therapy is facilitated greatly by the efforts of treatment centre nurse coordinators who are knowledgeable about haemophilia and its complications. Additional clinical trials will be necessary to define better initial dose, schedule, dose adjustment and success/failure criteria for IT.