M-mode echocardiography was performed on 107 young insulin-dependent diabetic subjects aged 2 – 24 yr (± SE: 13.8 ± 0.4 yr) and 636 age-group-matched controls. All patients were normotensive and free of cardiorespiratory symptoms. Diabetic patients showed a high prevalence of echocardiographic abnormalities that increased with age. Mean dimensions of the left atrium, right ventricle, and left ventricle (systolic and diastolic) were increased significantly in diabetic individuals (P < 0.01). Hypertrophy of the interventricular septum was present in some patients older than 12 yr of age. Mean interventricular septum excursion was markedly decreased in diabetic individuals compared with controls (3.9 ± 0.1 mm versus 5.6 ± 0.2 mm, respectively; P < 0.01). Fifteen percent of the diabetic patients but none of the controls had septal excursions less than 3 mm (2 SD below mean). Patients with decreased septal excursions showed a higher prevalence of other echocardiographic abnormalities than patients with normal septal excursions. Echocardiographic abnormalities did not correlate with either duration of diabetes or glucose control as assessed by hemoglobin Alc and plasma glucose concentrations at the time of echocardiographic testing. The results show a high prevalence of echocardiographic abnormalities in young diabetic subjects that may represent preclinical cardiomyopathy.