Aims/Hypothesis: Early determination of myocardial manifestations of diabetes mellitus is of major importance, since myocardial involvement considerably influences the prognosis of diabetic patients. The aim of this study was to investigate whether young patients with insulin-dependent diabetes mellitus and normal systolic left ventricular (LV) function already show a diastolic LV dysfunction and an increased risk of arrhythmias. Methods: Echocardiography was performed in 87 patients suffering from type I diabetes mellitus, without known cardiac disease and in 87 controls. Patients with a known manifest cardiac disease or a long-term diabetic syndrome were excluded. Morphological parameters were determined using M-mode echocardiography. Doppler echocardiography was used to evaluate parameters of LV diastolic function. The risk of arrhythmia was assessed by means of electrocardiography, heart rate variability, and late potential analysis. Results: The left atrial and ventricular dimensions and systolic functional parameters of all patients were normal. A diastolic dysfunction with a reduction in early diastolic filling, an increase in atrial filling, an extension of isovolumetric relaxation and deceleration time was documented in diabetic patients, as well as an increased number of supraventricular and ventricular premature beats. Conclusion: Even young patients with diabetes mellitus suffer from a diastolic dysfunction while systolic ventricular function is normal. Therefore, echocardiography with measurements of diastolic functional parameters appears to be a sensitive method for evaluating the manifestation and course of early diabetic cardiomyopathy.