Serum levels of angiotensin-converting enzyme (ACE) were measured in 53 patients with type II (non-insulin-dependent) diabetes (25 without ophthalmologic complications, 20 with background retinopathy, and eight with proliferative retinopathy) and in 33 healthy nondiabetic subjects. Diabetic subjects were excluded if they had hypertension, ischemic heart disease, peripheral vascular disease, or an elevated urine albumin level. After an overnight fast, blood was taken for determination of ACE, blood glucose, glycosylated hemoglobin (HbA1), and C peptide levels. Data were analyzed according to the nonpaired Student's t test and linear regression analysis. Levels of ACE were significantly elevated in the whole diabetic group as compared with control subjects (334.0 U/L +/- 97.0 vs 250.5 U/L +/- 85.5, P less than .001). This elevation was more marked in those diabetics with background retinopathy (344.6 U/L +/- 96.8, P less than .001) and proliferative retinopathy (357.3 U/L +/- 93.2, P less than .01); no significant difference was found between ACE levels of diabetics without complications and those of control subjects. No correlation was found between ACE levels and HbA1, blood glucose, or C peptide values. We conclude that ACE levels are elevated in type II diabetes, chiefly in patients with retinopathy. This finding may reflect microvascular damage caused by secretion of ACE by the vascular endothelial cells.