Anti-mitochondrial antibodies, the characteristic markers of primary biliary cirrhosis, have been detected in most patients with this disease. However, the prevalence of these antibodies in inflammatory myopathies and their clinical and histopathological significance has not been determined. Sera from 212 consecutive patients with inflammatory myopathies were screened for anti-mitochondrial antibodies by enzyme-linked immunosorbent assay. The clinical and histopathological features of anti-mitochondrial antibody-positive patients were analysed and statistically compared with those of anti-mitochondrial antibody-negative patients. Twenty-four patients positive for anti-mitochondrial antibodies (seven patients with and 17 patients without primary biliary cirrhosis) were identified (11.3%). Thirteen patients had a clinically chronic disease course of >12 months before their diagnosis at hospitals. Six of these 13 patients (four asymptomatic patients with increased creatine kinase levels and two patients with arrhythmia) had not been aware of muscle weakness, but all 13 patients had muscle atrophy at initial presentation. As complications, eight patients had cardiac involvement including arrhythmias (five patients with supraventricular tachycardia; two with ventricular tachycardia; and one patient with atrioventricular block), six patients had moderately decreased ejection fraction and six patients had decreased vital capacity, two of whom required respiratory support. Regarding muscle histopathological findings, in addition to inflammation, 13 patients had chronic myopathic changes and six had granulomatous lesions. Statistical analysis showed that the clinical features of a chronic disease course, cardiac involvement and muscle atrophy, and the histopathological features of chronic myopathic changes and granulomatous inflammation, were significantly more frequently observed in patients with anti-mitochondrial antibody-positive inflammatory myopathy than in patients who were negative for anti-mitochondrial antibodies. Except for cardiac involvement, which is more frequently observed in patients with primary biliary cirrhosis, no significant differences in clinical or histopathological features were found between patients with or without primary biliary cirrhosis. Our study revealed that inflammatory myopathies associated with anti-mitochondrial antibodies were frequently found in patients with the clinical features of a chronic disease course, muscle atrophy and cardiopulmonary involvement, and the characteristic histopathological feature of granulomatous inflammation. Our study suggests that inflammatory myopathies associated with anti-mitochondrial antibodies form a characteristic subgroup.