The no reflow phenomenon is reported to occur in >2% of all percutaneous coronary interventions (PCIs) and portends a poor prognosis. We analyzed data from 5,286 consecutive patients who underwent PCI from the Melbourne Interventional Group (MIG) registry from April 2004 through January 2008 who had 30-day follow-up completed. Patients without no reflow (normal reflow, n = 5,031) were compared to 255 (4.8%) with no reflow (n = 217 for transient no reflow, n = 38 for persistent no reflow). Patients with transient or persistent no reflow were more likely to present with ST-elevation myocardial infarction (MI) or cardiogenic shock (p <0.0001 for the 2 comparisons). They were also more likely to have complex lesions (American College of Cardiology/American Heart Association type B2/C), have lesions within a bypass graft, require an intra-aortic balloon pump, receive glycoprotein IIb/IIIa inhibition, and have a longer mean stent length (p <0.0001 for all comparisons). In-hospital outcomes were significantly worse in those patients with transient or persistent no reflow, with increased death, periprocedural MI, renal impairment, and major adverse cardiac events (p <0.0001 for all comparisons). Similarly, transient and persistent no reflow portended worse 30-day clinical outcomes, with a progressive increase in mortality (normal reflow 1.7% vs transient no reflow 5.5% vs persistent no reflow 13.2%, p <0.0001), MI, target vessel revascularization, and major adverse cardiac events (p <0.0001 for all comparisons) compared to patients with normal flow. In conclusion, transient or persistent no reflow complicates approximately 1 in 20 PCIs and results in stepwise increases in in-hospital and 30-day adverse outcomes.