A follow-up study was made of 29 patients aged 21 to 45 years, some 15-158 months after acute myopericarditis. The mean follow-up period was 72.9 months. The follow-up investigation included recording of history, physical examination, laboratory tests, radiologic examination of the heart and lungs and electrocardiography. All but one of the patients were fit for fulltime work. Nine had residual cardiac symptoms, but the physical examination was negative in all but 2 of them. One patient had chronic cardiac insufficiency and hepatic enlargement. Another had sinus tachycardia and cardiac enlargement of moderate degree and impaired working capacity in relation to heart size. Cardiac murmurs without clinical significance were audible in three cases. The resting ECG was pathologic in only 6 cases. Orthostatic ECG evoked ECG abnormalities in 6 more cases. Exercise tolerance tests showed reduced working capacity in relation to heart volume in 5 of the 29 cases (17%). Four of these 5 patients had cardiac enlargement. There was thus good correlation between increase in heart volume and reduction of physical capacity. The prognosis in regard to cardiac function was good, as was also found in other comparable series in which the observation time was somewhat shorter.