Sixteen cases of polymyositis (PM) were treated at the Central Hospital, Jyväskylä. Features of cardiac involvement were observed in 11 cases (69%). The heart diseases which developed or worsened after the onset of PM were grouped as follows: (1) Sole disturbance of conduction--one case, (2) congestive heart failure--four cases, (3) coronary heart disease with or without congestive heart failure--six cases. Three patients suffered an acute myocardial infarction. Electrocardiography revealed arrhythmias in eight cases and disturbances of conduction in three. In one patient complete atrioventricular block and congestive heart failure necesssitated installation of an intracardiac pacemaker. In four patients congestive heart failure progressed to death. In two autopsied cases changes suggestive of PM were found in the heart muscle. Involvement of the heart in PM was prognostically a bad sign.