Background: Obesity is a well-established risk factor for many chronic disorders. However, the effect of weight change after acute myocardial infarction (AMI) is not well known. Methods: Among consecutive patients who underwent percutaneous coronary intervention (PCI) between November 2005 and November 2007 due to acute myocardial infarction (AMI), patients who were overweight (23.0≤BMI<27.5 kg/㎡, n=341) and obese (BMI≥27.5 kg/㎡, n=80) were selected for analysis. According to weight change, patients were divided into 4 groups: Group I (weight loss>5%, n=61), Group II (0% weight loss≤5%, n=133), Group III (0%≤weight gain<5%, n=181), and Group IV (weight gain≥5%, n=46). We assessed association between weight change and major cardiac adverse events (MACE). Results: Greater weight loss was more frequent among older individuals (Group I: 64.1±12.4, II: 60.6±12.1, III: 59.0±11.9, IV: 61.4±10.6 years, p=0.028) and diabetics (Group I: 34.4%, II: 27.1%, III: 21.2%, IV: 15.2%, p=0.009). However, there were no significant differences in distributions of other baseline characteristics including hypertension, dyslipidemia, smoking, and gender among groups, or in angiographic and procedural factors including lesion characteristics, procedural success, target lesion location, number of stents used, and stent size, except for proportions of patients with three-vessel disease, which were higher in patients with weight loss (Group I: 20.8%, II: 23.0%, III: 12.5%, IV: 11.6%, p=0.005). The group with greater weight loss had the highest MACE rate at 12 month clinical follow-up (Group I: 36.9%, II: 25.0%, III: 25.9%, IV: 17.3%, p=0.020). Conclusions: Although weight loss after MI appears to be associated with worse outcomes, it remains unclear whether the effect is of cardiac origin. Further prospective study is needed to elucidate the effect of intentional weight reduction after MI.