Nephropathy is a major cause of premature morbidity and mortality in patients with non-insulin-dependent diabetes mellitus (NIDDM). The insertion/deletion (I/D) polymorphism of angiotensin-converting enzyme (ACE) is a genetic determinant of plasma ACE levels. Recent studies have found I/D polymorphism of the ACE gene to be associated with nephropathy in NIDDM. This association has not been evaluated in prospective studies. We, therefore, studied the relationship between ACE gene I/D polymorphism and diabetic albuminuria and glomerular filtration rate (GFR) in 83 NIDDM patients followed up for 9 years. At baseline, 29% (24 of 83) of the diabetic patients had an increased (>30 mg/24 h) urinary albumin excretion rate (UAER) and the prevalence of albuminuria at the 9-year examination was 35% (29 of 83). During the follow-up period, systolic blood pressure (p = 0.044), prevalence of hypertension (p < 0.01), and fasting blood glucose levels (p < 0.01) increased, while high-density lipoprotein cholesterol (p < 0.01) decreased. The declines of GFR during the follow-up period were 8.5, 14.1, and 16.3% within genotype groups of II, ID, and DD, respectively (p values for decreases: NS for II, <0.001 for ID, and <0.001 for DD). Patients with the DD genotype tended to have a steeper decrease of GFR, but the change was not statistically significant between the genotype groups. The increases of UAER during the follow-up period were 35.1, 8.3, and 122.4% within genotype groups of II, ID, and DD, respectively, but p values for all increases were not significant. Parallel to GFR, patients with the DD genotype tended to have a steeper increase of UAER, but the change was not statistically significant between the genotype groups. There were no differences in the ACE genotype distribution and allele frequencies between the patients with or without albuminuria either at follow-up or in cross-sectional settings. In conclusion, this 9-year follow-up study does not support the hypothesis that the ACE I/D polymorphism is a major genetic marker of diabetic nephropathy in NIDDM patients.