The safety and efficacy of a recombinant factor VIII (rFVIII) preparation (Kogenate) for the treatment of bleeding episodes was studied in previously untreated patients (PUPs) with severe, moderate, and mild hemophilia A. Patient peripheral blood samples taken at baseline and at 3, 6, 9, 12,18, and 24 months after the first infusion were evaluated for FVIII inhibitor antibodies by the Bethesda assay, for antibodies formed against trace proteins derived from the rFVIII production process, and for general changes in laboratory test results. Samples for general laboratory testing were also drawn every 6 months after the first 24 months. Hemostatic efficacy was assessed by physicians, and adverse events were recorded throughout the study period. Forty-three PUPs (30 with FVIII:C <1%; 10 with FVIII:C 1%-5%; and 3 with FVIII:C >5%) aged 3 months to 32 years were enrolled at 33 centers in Japan. Patients were studied for a mean of 51 months (range, 11-80 months), and the mean exposure time was 83 days (range, 2-571 days). The incidence of occurrence of FVIII inhibitors was 34.9% (high responders [≥10 Bethesda U/mL], 11.6%; low responders [0.5->10 Bethesda U/mL],23.3%). The median cumulative exposure time of inhibitor detection was 12 days, indicating inhibitor development at an early stage after the start of infusion of this preparation. Hemostasis was achieved with a single dose of Kogenate in 94.8% of the 951 bleeding episodes recorded in the study. Transient increases in antibodies against baby hamster kidney proteins and antimouse immunoglobulin G were observed in 14.0% and 18.6% of patients, respectively. Anti-rFVIII seroconversion was observed in 18.6% of patients and only in patients with inhibitor antibodies. Antibody responses to trace proteins were not correlated with drug-related adverse events with the exception of FVIII activity inhibition in PUPs with anti-rFVIII seroconversion. These data indicate that Kogenate is safe and effective for the treatment of bleeding in PUPs with hemophilia A.