Background. Extensive endothelial dysfunction (i.e., affecting many aspects of endothelial function) has been hypothesized to explain why microalbuminuria (MA) is associated with cardiovascular disease risk. However, it is not clear whether MA is specifically associated with impaired endothelial nitric oxide (NO) synthesis in individuals without and with type 2 diabetes. Methods. We did a population-based study in 645 individuals (mean age 68 years; 248 with normal glucose metabolism, 137 with impaired glucose metabolism, and 260 with type 2 diabetes) and investigated associations of MA [present (urinary albumin-creatinine ratio ≥2 mg/mmol) versus absent, and in four categories (<2, ≥2 to 5, ≥5 to 10, ≥10 mg/mmol)] with ultrasonically measured brachial artery endothelium-dependent, flow-mediated (FMD; an estimate of endothelial NO synthesis) and endothelium-independent, nitroglycerin-induced vasodilation (NID). Results. FMD was 0.12 mm in the presence of MA (N = 93; 49 with diabetes), and 0.18 in its absence (P = 0.002). After adjustment for age, sex, baseline arterial diameter, and other potential confounders, FMD was 0.038 mm (95% CI, 0.001 to 0.075) lower in the presence of MA (P = 0.04), and decreased linearly across MA categories [by 0.027 mm (0.007 to 0.046) per category increase of MA; P = 0.007]. NID was similar in individuals with and without MA. Results were similar in individuals without and with diabetes. Conclusion. Microalbuminuria is linearly associated with impaired endothelium-dependent, flow-mediated vasodilation in elderly individuals without and with diabetes. These findings support the concept that impaired endothelial nitric oxide synthesis plays a role in the association of microalbuminuria with cardiovascular disease risk. © 2004 by the International Society of Nephrology.