Abstract High-dose intravenous immunoglobulin (IVIG) therapy has been effective in treating many autoimmune and systemic inflammatory diseases. In the present prospective study, we evaluated the efficacy of IVIG for patients with polymyositis (PM) and dermatomyositis (DM) refractory to treatment with high-dose corticosteroids. PM/DM was defined as steroid-resistant when the muscle strength of a patient did not improve despite the administration of more than 50 mg prednisolone per day for more than 4 weeks. A total of 12 patients with biopsy-proven, steroid-resistant PM/DM received one infusion of polyethylene glycol-treated human IgG at a dose of 0.4 g per kg per day for five successive days. Three of the patients received a second infusion. All patients were followed for up to 3 months after the infusion. Finally, 8 patients (6 PM and 2 DM; 5 men and 3 women) aged 29-67 years (mean 48 years) were analyzed. Their clinical response was assessed by changes in (a) subjective signs, i.e., fatigue (visual analog scale, VAS), muscle pain (VAS), activities of daily living (ADL), (b) objective signs, i.e., manual muscle strength (MMT) and serum level of creatine kinase (CK). At 12 weeks after the infusion, the patients showed significant improvement in their scores of muscle strength (from a mean of 67.0 to 81.0) and their ADL scores (from a mean of 27.1 to 39.1). The mean serum CK level decreased significantly from 1287.4 to 612.6 IU/l. In addition, the mean VAS of fatigue decreased significantly from 5.5 to 1.3 cm. The physicians' assessment showed that 87.5% of patients had improved. The average reduced dose of prednisolone was 47.1 mg/day at 12 weeks after infusion in 7 patients who exhibited improvement. Adverse effects, i.e., asymptomatic myocardial infarction and increased blood urea nitrogen (BUN), were noted with two of the 15 infusion (13%). Overall, IVIG was found to be safe and effective for refractory PM and DM.