A 62-year-old woman who had coronary artery disease and hypertrophic cardiomyopathy received percutaneous transluminal coronary angioplasty and endomyocardial biopsy. She withstood the procedure well. However, delayed pericardial tamponade occurred 2 hours after discharge from the cardiac catheterization laboratory. Despite pericardial drainage with a pigtail catheter and blood transfusion, the patient required emergency surgery. An oozing diagonal branch of the left anterior descending coronary artery was found. Ligation of this small branch stabilized the hemodynamics. Avoidance of improper positioning of the guide wire in the small coronary artery branch is important in preventing arterial wall trauma and subsequent perforation.