Cardiac damage, consisting of mild diffuse myocarditis or severe inflammation and fibrosis of the conduction system or both, occurs in more than 70% of patients with idiopathic polymyositis. The lesions are strikingly similar to those detected in the infants of mothers with connective tissue diseases. In these infants the damage is associated with the transplacental passage of a maternal antibody to tissue ribonucleoproteins (anti-Ro). The same antibody was identified in 60% of 55 patients with polymyositis and in 69% (23/33) of those with associated cardiac damage including four with complete heart block. Forty five per cent of those patients who were anti-Ro seropositive had no clinical or electrocardiographic evidence of cardiac lesions. They were in the acute phase of illness, however, and no other more detailed heart investigations had been done. It is postulated that cardiac damage in polymyositis is caused by the antibody and that its presence may serve as a marker for heart involvement.