BACKGROUND AND OBJECTIVES: Stent thrombosis (ST) after a successful drug-eluting stent (DES) implantation has been reported in around 1% of clinical trials. However, the incidence of ST with a DES in practice is not well known. Here, we evaluated the possible causes of ST encountered in four Korean hospitals. SUBJECTS AND METHODS: Between March 2003 and December 2003, 450 patients were treated using a DES in the four study hospitals. We reviewed the clinical and procedural characteristics of 8 patients that experienced ST after a successful DES implantation. RESULTS: Eight patients (1.8%), who were administered antiplatelet medication, experienced ST, with a mortality of 50% at the 2 month follow-up. ST occurred within 7 days in all patients, with the exception of one, with an occurrence 37 days after intervention. Initially, all patients were diagnosed as having acute coronary syndrome. Direct PTCA was performed in 3 patients (38%). Severe calcification or a thrombus was noted in 6 (67%) of 9 lesions. In all cases, no GPIIbIIIa inhibitors were used prior to the DES implantation. Incomplete lesion coverage and incomplete stent expansion appeared in 7 (88%) cases. CONCLUSION: Our study demonstrated that full expansion of the DES and complete lesion coverage must be accomplished, with high pressure inflation or use of cutting balloon preferably under IVUS guidance, for the prevention of ST. Those patients with acute coronary syndrome might warrant intensive antiplatelet therapy, including GPIIbIIIa inhibitors.