Acute idopathic pericarditis can be accompanied by myocarditis, and in all types of acute pericarditis there are electrocardiographic signs of myocardial lesions. In order to determine the severity of the myocardial disease in acute idiopathic pericarditis, a prospective study has been carried out in a group of 25 patients with this diagnosis. The clinical evaluation included phonomecardiographic measurements of the systolic intervals, Weissler index in 24 cases, and echocardiographic study of the left ventricle in eight cases. Besides that, the serum levels of the myocardial enzymes (GOT, GPT, CPK, and LDH and its isoenzymes) were determined in all cases. The results showed a gallop rhythm in 8 percent of the cases, supraventricular arrhythmias in 4 percent, dysfunction of the left ventricle by systolic intervals and/or echocardiography in 32 percent, and increase of the myocardial enzymes in 24 percent, which represents a global incidence of myocardial disease of 44 percent. However, only three patients presented clinical manifestations of myocardial disease, although the congestive cardiac failure was always secondary to cardiac tamponade and not to myocardial failure. The increase of myocardial enzymes can determine important problems of differential diagnosis with an acute myocardial infarction. The clinical course was favourable in all of the cases, including the ones which showed myocardial disease.