The effects of high-dose intravenous immunoglobulin (i.v.Ig) in 12 MG patients were studied. All patients had severe symptoms. In two cases anti-acetylcholine receptor antibodies (anti-AChR abs) were not detectable. I.v.Ig was administered to 9 patients already on long-term immunosuppressive therapy and to 3 patients at the beginning of azathioprine treatment. 10 patients (83%) improved; the duration of improvement was longer in immunosuppressed patients. Anti-AChR abs generally decreased after infusion but we did not find a constant correlation between reduction in ab titers and clinical improvement. Side effects included one case of severe hemolysis. In our experience i.v.Ig therapy is effective in MG. The chief indication for its use appears to be the treatment of deterioration of the disease in patients already on immunosuppressive therapy.