Serious cardiac abnormalities and asymptomatic hyperkinetic heart function are frequently seen in patients with polymyositis/dermatomyositis (PM). The cardiac manifestations may be secondary to myocardial inflammation, replacement fibrosis, or alterations in the peripheral circulation. The significance of determinations of cardiospecific isoenzymes in the evaluation of myocardial manifestations in PM has not yet been established. We report on 2 patients with firm evidence of severe myocardial involvement. Nevertheless, the serum levels of creatine phosphokinase MB (CPK MB) and lactated dehydrogenase isoenzyme 1 (LDH1) were found normal. This finding casts doubt on the reliability of cardiospecific isoenzymes for the identification of inflammatory myocardial involvement in patients with PM.