Thirty-three children with type I diabetes mellitus and 51 normal children underwent M-mode echocardiography. Abnormalities of myocardial performance were present in many of the diabetic children. The mean end-systolic volume of the left ventricle was greater in diabetics compared to control subjects. Mean ejection fraction, minor axis shortening, and velocity of circumferential fiber shortening were decreased in the diabetics. There was no evidence of increased myocardial mass in these diabetic children. There was no correlation between myocardial dysfunction, clinical assessment of control, or glycohemoglobin in the diabetic children.