A case of inclusion body myositis masquerading as unresponsive polymyositis is presented. A 56-year-old woman diagnosed with "biopsy-proven" polymyositis in 1991 was referred to our clinic in 1997 with progressive, painless weakness that was unresponsive to steroid therapy. Further evaluation, including electromyography and review of the original muscle biopsy specimen, found a diagnosis of inclusion body myositis, leading to a change in the patient's prognosis and management. Inclusion body myositis is frequently mistaken for polymyositis, despite the fact that it is now the most common inflammatory myopathy affecting people older than 50 years. The purpose of this report is to increase awareness of this disease, to enhance early diagnosis, and to ensure appropriate management. We discuss the clinical findings, pathogenesis, and physiatric management, as well as compare this disease with other idiopathic inflammatory myopathies.