Background: Data regarding the impact of baseline platelet count on long-term outcomes of percutaneous coronary intervention (PCI) is limited. Methods: Baseline platelet count was available in 7,150 patients of total 7,221 patients received stent implantation between 1997 and 2003 in Asan Medical Center. Patients were divided into two category according to baseline platelet count; low level (platelet counts <150 x 10⁹/L), n=401 vs. normal level (platelet counts ��� 150 x 10⁹/L), n=6749. The rates of adverse outcomes were adjusted by means of proportional-hazards methods to account for differences in patients��� severity of illness according to level of platelet counts. Results: During 4-year follow-up, unadjusted freedom from all-cause mortality and stent thrombosis was significantly lower in patients with low level of platelet counts than in those with normal level of platelet counts (93.7% vs. 87.1%, p<0.001; 97.9% vs. 96.2%, p=0.02, respectively, Figure). After adjustment for baseline differences, the overall risk of all-cause death was consistently higher in patients with low platelet count (adjusted hazard ratio [HR], 1.68 ; 95% confidence interval [CI], 1.28 to 2.20 ; P<0.001), but adjusted risk of cardiac death in patients with low platelet count was not significant (P=0.18). In addition, adjusted risk for stent thrombosis was significantly higher in patients with low platelet count (HR, 1.79; 95% CI, 1.10 to 2.92; P=0.02). Conclusions: In this large cohort of PCI patients, baseline thrombocytopenia was significantly associated with increased risks of mortality and stent thrombosis.