Left ventricular (LV) function was assessed in 42 patients (mean age +/- SD, 18.45 +/- 3.76 years; 17 males) with type I diabetes mellitus (T1DM; mean duration 9.89 years) and in 43 healthy controls (mean age +/- SD, 18.27 +/- 3.36 years; 18 males). Systolic, diastolic cardiac function and LV dimensions were assessed using M-mode and Doppler echocardiography. Neural autonomic function was assessed by measuring RR variation during deep breathing, Valsava maneuver, 30/15 ratio, and blood pressure response to standing. Fractional shortening, peak velocity of early ventricular filling (E wave), peak velocity of LV filling (A wave), E/A ratio, deceleration time, isovolumic relaxation time, LV dimensions (interventricular septum, posterior wall thickness, end diastolic diameter [EDD] and systolic diameter [ESD]) were all comparable between patients with T1DM and controls. However, in 11 T1DM patients with microalbuminuria and/or retinopathy, EDD, ESD, E/A ratio, and E wave were all lower (p=0.0011, p=0.019, p=0.0011, and p=0.030, respectively) while, A wave, heart rate, and diastolic blood pressure were all higher (p=0.008, p=0.0024 and p=0.004, respectively) compared to matched for age and sex controls. Furthermore, in six of the 11 T1DM patients with microangiopathy who had E/A <1.12 (<2 SD of the control mean), significant and marginally significant correlations were found between E/A ratio and the duration of the disease as well as the mean HbA1c of the last year (r=-0.38, p=0.011 and r=-0.287, p=0.064, respectively). In conclusion, it has been found that impairment of diastolic, but not systolic, LV function can be detected early in young patients with T1DM and microangiopathy.