There has been growing evidence associating obstructive sleep apnea (OSA) with cardiovascular pathogenesis. We hypothesized that OSA may affect outcomes after percutaneous coronary intervention (PCI) in patients with acute coronary syndrome (ACS). We performed a sleep study in 89 consecutive patients with ACS who were successfully treated with PCI. Patients with an apnea hypopnea index > or =10/hour were considered to have OSA. Co-morbidity of OSA with ACS was found in 51 patients (57%). There were no differences in baseline demographics between patients with and without OSA, except for significantly higher high-sensitivity C-reactive protein levels (0.59 +/- 0.75 vs 0.29 +/- 0.20 mg/dl, p = 0.019) in patients with OSA. Patients were followed for a mean period of 227 days. The incidence of major adverse cardiac events (cardiac death, reinfarction, and target vessel revascularization) was significantly higher in patients with OSA (23.5% vs 5.3%, p = 0.022). By multivariate analysis, the presence of OSA was an independent predictor for major adverse cardiac events (hazard ratio 11.61, 95% confidence interval 2.17 to 62.24, p = 0.004). In addition, quantitative coronary angiography at 6-month follow-up depicted significantly greater late loss (1.28 +/- 0.84 vs 0.69 +/- 0.81 mm, p = 0.003) and a higher binary restenosis rate (36.5% vs 15.4%, p = 0.026) in patients with OSA compared with those without OSA. In conclusion, the present study showed a high prevalence of OSA among patients with ACS. Moreover, OSA appeared to be an independent predictor for clinical and angiographic outcomes after PCI.