A 50-year-old female with a malfunctioning bioprosthetic mitral valve which was implanted 7 years earlier underwent reoperation. She had no history of angina pectoris. Calcium channel blockers and nitrates had not been taken by the patient. Coronary arteriograms were normal. About 2 hours after the reoperation, EKG monitor showed abrupt ST segment elevation which was immediately followed by ventricular tachycardias and fibrillations. This life threatening circulatory collapse recurred until a drip infusion of diltiazem was started. Maximum CPK-MB was 145 IU/l. Postoperative coronary arteriography, which was performed after 12 hours of withhold of diltiazem, showed a spasm in the proximal segment of the right coronary artery. Coronary artery spasm should be considered among the causes of abrupt and unexpected circulatory collapse after open heart surgery even in the absence of coronary artery disease.