Arrhythmias and even sudden death may be caused by acute infectious myocarditis. We therefore recorded 24-hour electrocardiograms in 26 myocarditis patients both one week and two to three months after the detection of myocarditis. Two of the patients were also monitored on the first day of their symptomatic myopericarditis . Twenty-eight control subjects had recordings one to two weeks after the onset of acute uncomplicated infections. One of the two patients monitored during the first day of myopericarditis had repetitive, asymptomatic bursts of ventricular tachycardia. In the 1-week recordings 27% of the myocarditis patients and 7% of the control subjects (p less than 0.06) had complex ventricular premature beats; these occurred in only 8% of the myocarditis patients during the late recording. However, the overall number of ventricular premature beats was low in both groups and recordings. A lengthening of the QT interval was related (p less than 0.05) to the occurrence of complex premature beats in repeated recordings in the same individuals. The heart rate was higher (p less than 0.02) in myocarditis patients, but no significant difference was noted in the frequency of conduction defects. We conclude that there seems to be no reason for a prolonged restriction of physical activity during clinically uncomplicated recovery from mild acute myocarditis. However, potentially dangerous arrhythmias may occur during the very first days of the disease.