A total of 161 infants and children, ranging in age from 1 day to 17 years at initial encounter (mean, 3.7 years), was seen over a 30 year period with primary myocardial disease (idiopathic myocarditis, nonobstructive cardiomyopathy, endocardial fibroelastosis, and an anatomically unknown category). These patients were observed from 1 hour to 23 years after initial encounter and cardiac disease has resolved in 27 per cent, resulted in death in 35 per cent, and continues in 38 per cent. The majority were first referred to us with congestive heart failure; all exhibited ST-T changes and cardiomegaly, 67 of 150 had left ventricular hypertrophy, 23 of 151 arrhythmias, and 55 of 153 pulmonary vascular congestion. Initial ventricular depolarization abnormalities were very frequent. Significant clinical predictors of fatal outcome included pulmonary vascular congestion, "northwest" axis deviation, and a cardiac index less than three L./min./M.2. Death occurred during the first year after initial encounter in 44 of 57 who died, and in all 13 with proved myocarditis. Primary myocardial disease is a serious disease of infancy and childhood, resulting in death or residual cardiac disease in three fourths of those affected.