The high incidence of cardiac mortality in type 1 diabetic patients is further increased when diabetic nephropathy is present. Since microalbuminuria (albumin excretion rate > 30 mg/day) represents an incipient stage of diabetic nephropathy, we decided to investigate whether incipient renal changes correlate with early diastolic cardiac dysfunction, known to preceed systolic dysfunction. To test this hypothesis, 13 normotensive type 1 diabetics with incipient nephropathy (mean age 37.8 ± 3.5, diabetes duration 18.8 ± 2.6 years), 13 type 1 diabetics without nephropathy (matched for age, diabetes duration and metabolic control) or other microangiopathic changes and 13 normal controls were studied. Diseases known to affect left ventricular performance were ruled out before investigation. Right and left ventricular parameters were assessed by M-mode and Doppler echocardiography. While parameters for left ventricular systolic function stayed within the normal range and did not differ between the two diabetic groups (ejection fraction 69.3 ± 2.4 vs. 69.6 ± 1.4%; fractional shortening 40.3 ± 2.2 vs. 38.5 ± 1.1%), diastolic function was significantly impaired in diabetic patients with microalbuminuria. This is expressed by an inversed early and late peak flow velocity ratio in patients with microalbuminuria (0.988 ± 0.04 vs. 1.362 ± 0.1; p < 0.05) and a significant percentual increase in late (atrial) filling (39.1 ± 1.7 vs. 29.9 ± 1.4% when compared to whole filling; p < 0.05) despite a similar rate-corrected isovolumetric relaxation period in both diabetic groups (98.4 ± 5.8 vs. 95.5 ± 6.2). The data give evidence for the presence of a significant diastolic dysfunction in diabetic patients with microalbuminuria in contrast to diabetic patients without nephropathy and normal controls.