Background: Microalbuminuria (MA) is considered a major risk factor predisposing to cardiovascular morbidity and mortality. Outcomes after percutaneous coronary intervention (PCI) for patients with acute myocardial infarction (AMI) complicated by MA have been well described. However, data regarding admission MA and coronary and myocardial flow are scant. The aims of this study were to evaluate the effects of admission MA on coronary blood flow and prognosis in ST-segment elevation myocardial infarction (STEMI) patients undergoing primary PCI. Hypothesis: Did elevated admission microalbuminuria predict poor myocardial blood flow and 6-month mortality in ST-segment elevation myocardial infarction patients undergoing primary percutaneous coronary intervention? Methods: A total of 247 patients undergoing primary PCI for STEMI within 12 hours after symptom onset were studied. Patients were divided into 2 groups according to admission urinary albumin extraction rate (UAER): (1) an MA group (UAER 20-200 mu g/min), and (2) a normoalbuminuria (NA) group (UAER < 20 mu g/min). Results: Microalbuminuria was observed in 108 patients. Univariate analyses showed statistical differences between the NA and MA groups in serum creatine level, plasma glucose level, and peak creatine kinase level on presentation. Thrombolysis In Myocardial Infarction (TIMI) flow grades (TFGs) 0-2 in the MA group were more frequent (9.4% vs 21.2%, P < 0.05) than in the NA group, and corrected TIMI frame count was higher (23.9 +/- 18.5 vs 29.8 +/- 23.5, P < 0.05). Admission MA was an independent predictor of poor myocardial perfusion (adjusted relative risk: 3.14, 95% confidence interval: 0.99-6.78) and a higher rate of 6-month mortality in STEMI patients undergoing primary PCI (adjusted relative risk: 1.58, 95% confidence interval: 0.74-3.39). Conclusions: Admission MA levels are associated with impaired myocardial flow and poor prognosis in STEMI patients undergoing primary PCI.