We have previously shown impaired ventricular function in asymptomatic middle-aged type 1 (insulin-dependent) diabetic patients who had no evidence of coronary artery disease. The diabetic patients had normal coronary angiograms but reduced ventricular ejection fraction on exercise. To examine the possible contribution of small vessel disease to this functional abnormality, we compared endomyocardial biopsies from seven symptom-free type 1 diabetic patients with biopsies from seven age- and sex-matched non-diabetic subjects. Interstitial fibrosis was present in three diabetic patients, arteriolar hyalinization in three patients and arteriolar thickening was observed in five patients. Morphometry performed on electron micrographs showed no significant difference in the thickness of the capillary basal lamina between diabetics and controls. While the functional significance of the abnormalities on light microscopy is unclear, our findings indicate that the abnormality of cardiac function described in diabetes is not associated with thickening of the myocardial capillary basal lamina.