Left ventricular thrombus (LVT) is a possible complication, especially in patients who experience acute myocardial infarction (MI), even after revascularization. We encountered a case of unrecognized MI an LVT in a 38-year-old man, which was discovered incidentally on electrocardiogram. The patient experienced no symptoms, such as chest pain or dyspnea, and had no relevant medical history or regular medication use. However, he did have a history of smoking (15 cigarettes daily), and his father had MI. The patient was obese and his low-density lipoprotein cholesterol levels were high (170 mg/dL). In addition to pharmacotherapy for secondary prevention of MI, anticoagulation therapy with warfarin was started, and percutaneous coronary intervention of the left anterior descending artery was successful. Management of cardiovascular risk is important even at a young age.