Electrocardiographic abnormalities were found in 25 (32.5%) of 77 patients with polymyositis. Left anterior hemiblock (13.0%) and right bundle-branch block (9.1%) were the most common abnormalities. No association could be found between these defects and clinical activity of polymyositis, severity of duration of disease, or degree of creatine kinase level elevation. Similarly, there was no association with age or the presence of certain clinical conditions such as rash or arthritis. In only two patients, both children, did an ECG abnormality improve with corticosteroid therapy. Three patients with heart block died suddenly. This study points up the need for continual close observation of patients with inflammatory muscle disease, even after apparent remission.