Thirty-six patients (15 females, 21 males) with inclusion body myositis (IBM) were studied. The diagnosis was established according to clinical and histopathological criteria. Clinical features were insidious onset of slowly progressive muscle weakness and wasting with depressed or absent tendon reflexes especially in the lower limbs. The pattern of muscle weakness was variable. The majority of patients (58%) showed proximal and symmetrical weakness usually most prominent in the legs. Isolated distal (6%) and asymmetrical weakness (19%) was less frequently observed. Myalgia occurred in 42% of the patients. The age at onset of symptoms ranged from 20 to 73 years (mean 47 years). Serum creatine kinase levels were normal (11%) or mildly elevated (89%). Needle electromyography revealed myopathic features in about 80% of the patients, and results of nerve conduction studies were normal in most of the cases. The predominant histopathological findings were numerous muscle fibers with rimmed vacuoles (100% of the patients), groups of atrophic fibers (92%), and inflammatory infiltrates (89%). The inflammatory infiltrates were located predominantly at endomysial sites and were composed mainly of T8 cells. Electron microscopy showed characteristic intracytoplasmic filamentous inclusions in all 36 cases. Immunosuppressive treatment in 16 patients failed to prevent disease progression in all but one patient with an associated Sjögren's syndrome. It is concluded that the consistent combination of typical histopathological findings and characteristic clinical features offers a firm basis for the diagnosis of IBM. IBM should be suspected in any adult patient presenting with clinical signs of a chronic polymyositis unresponsive to immunosuppressive therapy. The etiology and pathogenesis of IBM remain to be established.