Transluminal coronary angioplasty may be associated with complications resulting from the dilatation catheter and inflation of the balloon. The most common complications are dissection, occlusion, and coronary spasm. We report an unusual complication of coronary artery perforation by the dilatation catheter resulting in acute pericardial tamponade. The complication was immediately recognized and confirmed by dye injection and hemodynamic measurements. Pericardiocentesis was performed, followed by successful coronary bypass surgery. The reasons for the perforation are unclear. We postulate that the acute angle of the perforated vessel was an important factor for this complication. The importance of a standby cardiac surgeon and operating room is emphasized.