The left ventricular function was assessed in 70 diabetics under 60 yr of age without clinically evident heart disease using established non-invasive methods; these include systolic time interval method and echocardiography. The ratio of pre-ejection period to left ventricular ejection time (PEP/ET) was remarkably elevated in diabetics with severe microangiopathy (0.43 .+-. 0.037). Even in diabetics without microangiopathy PEP/ET ratio was significantly higher (0.374 .+-. 0.037) compared with that in controls (0.331 .+-. 0.023, P < 0.01). Most diabetics with a PEP/ET value > 0.40 were not under proper care with regard to diabetic control. A tendency toward normalization of PEP/ET values was often observed with the improvement in diabetic control during 6-12 mo. among the inadequately controlled diabetics. Isovolumic relaxation time in diabetics was longer than in controls (80 .+-. 14 ms, 59 .+-. 11 ms, P < 0.005). Abnormalities of left ventricular function in diabetics may be related to not only severity of microangiopathy but also the state of diabetic control. The maintenance of adequate control of diabetes seems to play an important role in the prevention of congestive heart failure in patients with diabetes mellitus.