Objective: Anti-mitochondrial antibodies (AMAs) can be detected in some idiopathic inflammatory myopathy (IIM) patients. We aimed to investigate the clinical features of IIM patients with AMAs. Methods: We retrospectively analysed 1,167 consecutive patients with IIM for AMA-associated myositis and compared them to age-and gender-matched AMA-negative IIM patients. Results: Twenty-nine patients (2.5%) were identified with AMA-positive myositis; eight of them had primary biliary cholangitis (PBC). There were no significant differences in skin rash, dysphagia, interstitial lung dis-ease, and muscle strength between AMA-positive patients and AMA-negative patients. Of 23 cases, 12 (52.2%) showed immune-mediated necrotizing myopathy (IMNM)-like pathological features. amongst AMA-positive patients, 11 of 16 patients with isolated anti-AMAs were classified as IMNM which was significantly higher than that of patients with coexistent anti-AMAs and myositis-specific antibodies (p = 0.026). More -over, subclinical cardiac involvement was significantly more common in AMA-positive patients than in AMA-negative patients (21/29 VS 33/116, p<0.001). In addition, patients without PBC had a significantly higher incidence of abnormal echocardiography findings than that of patients with PBC (p = 0.009). Patients without heart abnormalities took significantly less time to achieve disease remission and prednisone taper-ing to <10 mg than patients with heart abnormalities (p = 0.000 and p = 0.001, respectively). Conclusions: IMNM was a major histopathological finding in IIM patients with isolated AMAs. AMAs were sig-nificantly associated with subclinical cardiac involvement in IIM. PBC seemed to be a protective factor for abnormal echocardiography findings in AMA-positive patients. Patients without heart involvement took less time to achieve disease remission and prednisone tapering off. (c) 2021 Elsevier Inc. All rights reserved.