Background: Microalbuminuria is associated with atherosclerosis and it is a strong and independent predictor of increased risk for cardiovascular morbidity and mortality. However, the underlying mechanisms of the association of albuminuria and cardiovascular disease are not well understood. We examined the association of endothelial dysfunction with microalbuminuria in non-diabetic and non-hypertensive patients with acute coronary syndromes (ACS). Methods: We compared endothelial function by flow-mediated dilatation (FMD) and glyceryl trinitrate-mediated dilatation (GTN) and angiographic properties among 27 microalbuminuric and 106 normoalbuminuric patients. Severity of coronary arteriosclerosis was evaluated using the Gensini score and number of diseased vessels. Results: We evaluated 133 patients (106 males; mean age 59 +/- 11 years). Microalbuminuria was present in 27 (20.3%) subjects. Patients with microalbuminuria and normoalbuminuria had similar baseline characteristics. FMD and GTN responses were not different in microalbuminuric patients compared with normoalbuminuric patients (FMD 8.2 +/- 5.3 vs. 7.9 +/- 6.5%, p = 0.54 and GTN 9.3 +/- 5.4 vs. 10.2 +/- 6.9%, p = 0.82). Microalbuminuria was not associated with endothelial dysfunction (p = 0.49). Morphological properties of coronary lesion were not different. Conclusion: The presence of microalbuminuria is not associated with endothelial dysfunction and severity of angiographic coronary atherosclerosis in non-diabetic and non-hypertensive patients with ACS.