Background: Restenosis after percutaneous coronary intervention(PCI) has been thought to present in a stable manner as exertional angina. However, the presentation of in-stent restenosis(ISR) is not well-studied. We hypothesized that a substantial proportion of bare metal ISR presents as acute coronary syndromes. We aimed to characterize the severity of the clinical presentation of ISR. Methods: We searched our PCI database for all cases of PCI for bare metal ISR occurring between May 1999 and September 2003. Multivessel interventions were excluded. In-stent restenosis presentation was classified into three categories:(1) myocardial infarction(MI),(2) unstable angina requiring hospitalization before angiography, and(3) exertional angina. Routine angiographic screening after initial stent placement was not performed, so ISR episodes were clinical, rather than angiographic, ISR. Results: We identified 1186 cases of bare metal ISR in 984 patients. Median age was 63, 72%were male, and 36%had diabetes. Of the ISR episodes, 9.5%presented as acute MI(7.3%as non-ST-segment elevation MI and 2.2%as ST-segment elevation MI), 26.4%as unstable angina requiring hospitalization before angiography, and 64.1%as exertional angina. Conclusions: More than one third of bare metal ISR episodes presented as MI or unstable angina requiring hospitalization. The acuity of the clinical presentation of bare metal ISR appears to be more severe than has been previously thought. Aggressive efforts, such as drug-eluting stents to decrease the incidence of unstable angina due to bare metal ISR, are warranted. Background: Restenosis after percutaneous coronary intervention (PCI) has been thought to present in a stable manner as exertional angina. However, the presentation of in-stent restenosis (ISR) is not well-studied. We hypothesized that a substantial proportion of bare metal We aimed to characterize the severity of the clinical presentation of ISR. Methods: We searched our PCI database for all cases of PCI for bare metal ISR occurring between May 1999 and September 2003. Multivessel interventions were excluded. In -stent restenosis presentation was classified into three categories: (1) myocardial infarction (MI), (2) unstable angina requiring hospitalization before angiography, and (3) exertional angina. Routine angiographic screening after initial stent placement was not performed, so ISR episodes were clinical, rather than angiographic, ISR. Results: We identified 1186 cases of bare metal ISR in 984 patients. Median age was 63, 72% were male, and 36% h Of the ISR episodes, 9.5% presented as acute MI (7.3% as non-ST-segment elevation MI and 2.2% as ST-segment elevation MI), 26.4% as unstable angina requiring hospitalization before angiography, and 64.1% as exertional angina. Conclusions: More than one third of bare metal ISR episodes presented as MI or unstable angina requiring hospitalization. The acuity of the clinical presentation of bare metal ISR appears to be more severe than has been previously thought. Aggressive efforts, such as drug -eluting stents to decrease the incidence of unstable angina due to bare metal ISR, are warranted.