Background: While coronary artery aneurysm is an uncommon anatomic disorder and has various forms, its clinical features and outcome and its impact on thrombus formation and the no-reflow phenomenon in the clinical setting of acute myocardial infarction (AMI) undergoing primary percutaneous coronary intervention (p-PCI) have not been discussed. The purpose of this study was to evaluate whether this anatomic disorder predisposes to a high burden of thrombus formation, and subsequently leads to the no-reflow phenomenon and untoward clinical outcome in patients with AMI undergoing p-PCI. Methods and Results: In our hospital, emergency p-PCI was performed in 924 consecutive patients with AMI between May 1993 and July 2001. Of these 924 patients, 24 patients (2.6%) who had an infarct-related artery (IRA) with aneurysmal dilatation were retrospectively registered and constituted the patient population of this study. Angiographic findings demonstrated that the ectasia type (defined as diffuse dilatation of 50% or more of the length of the IRA) was found most frequently (70%), followed by the fusiform type (20%; defined as a spindle-shaped dilatation in the IRA) and the saccular type (10%; defined as a localized spherical-shaped dilatation in the IRA). The right coronary artery was the most frequently involved vessel (54.2%), followed by the left anterior descending (25.0%) and the left circumflex arteries (20.8%). Coronary angiography revealed that all of these aneurysmal IRA filled with heavy thrombus (indicated as high-burden thrombus formation). The no-reflow phenomenon (defined as ≤TIMI-2 flow) and distal embolization after p-PCI were found in 62.5 and 70.8% of the IRA, respectively. The incidence of cardiogenic shock and the 30-day mortality rate were 25 and 8.3%, respectively. The survival rate was 90.9% (20/22) during a mean follow-up of 19 ± 30 months. Conclusions: While aneurysmal dilatation of an IRA is an uncommon angiographic finding in the clinical setting of AMI, it is frequently associated with high-burden thrombus formation and has a significantly lower incidence of successful reperfusion. However, the long-term survival of these patients is excellent.