Echocardiographic assessment of cardiac function was made on 24 children with chronic renal failure of varying etiology and severity. In 20 patients without evidence of cardiac failure, parameters of left ventricular performance as represented by PEP/LVET and mean velocity of circumferential fiber shortening were within normal limits in the majority of patients. In addition, ejection fraction and shortening fraction were, in most children, within the 95% confidence limits for their age. In 4 patients who presented with congestive heart failure, marked left ventricular dilatation was noted in association with decreased shortening and ejection fractions and depressed mean velocity of circumferential fiber shortening. Also the PEP/LVET in these patients suggested the presence of a uremic cardiomyopathic condition. These studies, in addition to our own studies on children who have undergone fistula construction, hemodialysis, and transplantation, suggest that cardiac performance, in the majority of pediatric patients with end-stage renal disease, is well maintained and that the major factor involved in reducing exercise tolerance is the presence of uremic anemia. Only a minority of patients may develop severe uremic heart disease.