BACKGROUND Baseline vessel patency is known to be associated with outcomes after primary percutaneous coronary intervention (PCI) in ST-segment elevation myocardial infarction (STEMI). However, the prevalence of coronary occlusion and impact of baseline Thrombolysis In Myocardial Infarction (TIMI) grade on mortality in non-ST-segment elevation myocardial infarction (NSTEMI) is not well known. METHODS From October 2005 to May 2007, 8,565 patients completed registration in the Korean Acute Myocardial Infarction Registry (KAMIR). NSTEMI was present in 3,372 patients (39.4%) and STMEI in 5,074 patients (59.2%). We evaluated an impact of baseline TIMI grade on 1-month mortality in 1,168 NSTEMI patients treated with early invasive strategy. RESULTS In NSTEMI patients treated with early invasive strategy, baseline TIMI grade was as follows: grade 0 in 355 patients (30.4%); grade 1 in 153 (13.1%); grade 2 in 177 (15.2%); and grade 3 in 483 (41.4%). Baseline characteristics were not significantly different between patients with baseline TIMI grade 0 to 1 flow (group I) and those with baseline TIMI grade 2 to 3 flow (group II) except age (60 years in group I versus 64 years in group II, p = 0.001). Patients in group I were significantly more likely to have a left circumflex (LCx) artery culprit lesion and less likely to have left anterior descending (LAD) artery involvement compared with group II (LCx: 34.5% versus 23.0% and LAD: 34.3% versus 48.2%, p <0.001). Total mortality at 1-month was 3.3%. Patients in group I demonstrated a higher 1-month mortality than those in group II (4.7% versus 2.1%, p = 0.01). In multivariate analysis, baseline TIMI grade 0 to 1 flow was an independent predictor of 1-month mortality (OR 2.87, 95% CI 1.33 to 6.18, p = 0.006) with age (OR per 10 years 1.76, 95% CI 1.18 to 2.62, p = 0.006) and Killip class (OR 5.82, 95% CI 2.71 to 12.47, p < 0.001). CONCLUSIONS In NSTEMI patients treated with early invasive strategy, baseline TIMI grade is an independent predictor of 1-month mortality.