Background: Guidelines recommend early invasive strategy for non-ST-segment elevation myocardial infarction (NSTEMI), but optimal timing of intervention is uncertain. There is no evidence of clinical superiority of immediate intervention over the deferred strategy. Additionally, recently reported 5-year follow-up from the ICTUS trial shows that even patients with elevated troponin may not benefit substantially from early invasive strategy compared with conservative strategy. Methods: A cohort of 4,929 patients with NSTEMI were selected from a prospective cohort study, the Korea Acute Myocardial Infarction Registry, and stratified according to initial treatment strategy and timing of intervention after presentation and analyzed for death and composite of major adverse cardiac events (MACE, death or myocardial infarction or revascularization) at 12-month. Results: Percutaneous coronary intervention (PCI) was performed in 3,584 (73 %) patients (median age 63 years, 68.7% male) with NSTEMI at median of 26.1 hr after presentation (���12 hr [n = 936], 12 to 24 hr [n = 643], 24 to 48 hr [n = 671], 48 to 72 hr [n = 431], and 72 hr to 30 days [n = 724]) with significantly better 12-month MACE, compared to medical therapy (11.1% vs. 28.3%, p <0.0001). Among invasive strategy, timing of PCI was significantly associated with 12-month composite of MACE (12.9%, 10.6%, 12.4%, 7.2%, and 10.5%, respectively; p = 0.041) with a linear association. In multivariable analysis, timing of PCI was a significant independent predictor of 12-month composite of MACE after adjusting significant factors, age (OR 1.02, 95% CI 1.01-1.03, p = 0.0003), pulmonary edema or cardiogenic shock (OR 2.62, 95% CI 1.93-3.56, p <0.0001), and TIMI risk score (OR 1.41, 95% CI 1.11-1.78, p = 0.0045). The adjusted ORs for 12-month composite of MACE in ���12 hr, 12 to 24 hr, 24 to 48 hr, and 48 to 72 hr groups, compared with that in 72 hr to 30 days group, were 1.57 (p = 0.0050, 95% CI 1.15-2.16), 1.29 (p = 0.16, 95% CI 0.90-1.85), 1.52 (p = 0.017, 95% CI 1.08-2.14), and 0.78 (p = 0.27, 95% CI 0.50-1.22), respectively. In NSTEMI patients with high TIMI risk score ���5, timing of PCI was not a significant predictor of 12-month composite of MACE (p = 0.56). Conclusions: In this real-world cohort of patients with NSTEMI, we demonstrated the benefit of invasive strategy in reducing 12-month composite of MACE. Delaying timing of PCI >24 hr was not an independent predictor of 12-month composite of MACE. These findings suggest that, in most patients with NSTEMI, urgent PCI is not mandatory and timing of PCI can be flexible and determined on an individual basis, depending on the patient���s risk profile and clinical course.