A decrease in inhibitor titer has been reported in some patients with acquired factor VIII inhibitors treated with intravenous gammaglobulin (IGIV). We have conducted a prospective, multicenter study of high-dose IGIV in treatment of this disorder to determine efficacy. Nineteen patients received induction therapy with IGIV, 1,000 mg/kg x 2 consecutive days or 400 mg/kg x 5 consecutive days, followed by maintenance doses at intervals as clinically indicated. A > or = 25% reduction in inhibitor titer was observed in 8 of 16 assessable patients, six of whom met the criteria for response. A rapid decline in inhibitor titer over 3 to 4 days was observed in two patients, but in four additional responding patients--two of whom received repetitive maintenance doses of IGIV--the decline was more gradual, with final nadirs being reached several weeks to many months after treatment. The inhibitor disappeared in three patients with low-level inhibitors, less than 1 Bethesda unit (BU). Concomitant therapy with prednisone may have contributed to the response in two of the patients but was not a factor in four patients; the response rate is, therefore, estimated to be between 25% and 37.5%. These results confirm the beneficial effect of IGIV in treatment of some patients with acquired autoimmune factor VIII inhibitors.