The clinical profile and course of documented cases of idiopathic dilated cardiomyopathy in children have been poorly characterized. Twenty-four patients (median age 2 years, range less than 1 month to 18 years) with idiopathic dilated cardiomyopathy were identified from Mayo Clinic records from 1973 to 1982. The most common presentation was congestive heart failure (92% of patients). Echocardiography (22 patients) generally revealed a dilated left ventricle with reduced fractional shortening (mean 14%) and ejection fraction (mean 26%). Two-dimensional echocardiographic evidence of left ventricular thrombus was present in 3 (23%) of 13 patients. Median cardiac index and left ventricular end-diastolic pressure (19 patients) were 2.5 liters/min per m2 and 22 mm Hg, respectively. Myocardial biopsy in eight patients showed nonspecific findings without active inflammation or evidence of endocardial fibroelastosis. On follow-up (mean duration 33 months, range 0 to 149), systemic arterial embolism had occurred in 2 (8%) of 24 patients. Fifteen of 24 patients had died (63% survival at 1 year and 34% survival at 5 years of follow-up). The cause of death was congestive heart failure in 11, complications after cardiac transplantation in 3 and sudden cardiac death in 1. Nine patients are alive at a mean follow-up time of 65 months (range 26 to 149); five are asymptomatic. Serial determination of left ventricular systolic function, available in all survivors, showed improvement in six patients and no significant change in three. Severe mitral insufficiency was present only in patients who ultimately died. A recent viral syndrome was noted more frequently in patients who survived.(ABSTRACT TRUNCATED AT 250 WORDS)