A 52-year-old man with diagnosis of post-infarction unstable angina. Coronary angiography revealed 90% luminal obstruction in the middle third of the right coronary artery and 90% in the marginal branch of the circumflex artery. After the administration of clopidogrel 300 mg associated with acetylsalicylic acid, the patient underwent the implantation of a sirolimus-eluting stent (CYPHER; Johnson & Johnson Cordis) 2.5 x 1.8 mm in the lesion located at the left marginal branch. One year and five months after the CYPHER stent implantation, a new angiography showed intrastent coronary aneurysm at the left marginal branch. This case report suggests that the implantation of a sirolimus-coated coronary stent may cause late formation of coronary artery aneurysm.