Biopsy specimens from the myocardium were examined in a series of 145 patients who had elected coronary arterial bypass grafting. The patients were divided into three groups; 1) overtly diabetic (OD) patients; 2) chemically diabetic (CD) patients, who demonstrated impaired glucose tolerance only when stressed with a sugar load; and 3) normoglycemic, nondiabetic (ND) patients, who served as a control group. Tissue plugs from the left anterior apical segment of the heart and from the quadriceps femoris in 71 patients, for comparative evaluation, were prepared for ultrastructural examination. Findings were as follows: 1) Myocardial hypertrophy and interstitial fibrosis were twin characteristic abnormalities, seen in all but two of the biopsy specimens; capillary endothelial changes, the third most common abnormality, were present in approximately half of these specimens, regardless of the patients' metabolic status. 2) In patients matched by sex, age, weight, blood pressure, preoperative myocardial ventricular function, and coronary arterial integrity, capillary basal laminar thickening represented a pathomorphologic hallmark, distinguishing structural alterations in the diabetic from those in the normoglycemic patient. 3) Although clear-cut and statistically significant thickening of basal laminae was noticeable in OD patients, a) in the quadriceps markedly increased laminar thickening was present in a number of ND patients, rendering interpretation of this change in skeletal muscle as pathognomonic for diabetes doubtful; and b) within cardiac muscle this increase in laminar width was less than that seen in skeletal muscle, leaving the functional implications of this alteration in doubt. 4) Early but statistically significant increases in capillary basal laminar thickening were observed in the myocardium of CD patients; these patients demonstrated impaired glucose tolerance only when stressed with a sugar load, without exhibiting overt diabetic manifestations. 5) In this group of highly selected patients with epicardial coronary arterial disease, the histopathologic profile of the diabetic myocardium did not include distinctive abnormalities sufficient to warrant the designation of "diabetic cardiomyopathy," indicating that coronary arterial bypass grafting can be recommended for the diabetic patient who requires this procedure.