A fifty-four-year-old woman was admitted to the hospital for a sensation of tightness in the chest of one hour's duration. She had undergone surgery for breast cancer two years previously and had been taking 30 mg of tamoxifen and 1200 mg of medroxyproges terone daily after surgery. Emergency coronary angiography on admission revealed thrombi in both the right coronary artery and the left anterior descending coronary artery. Tissue-type plasminogen activator was injected into both coronary arteries, resulting in diminution of thrombus size. Repeat coronary angiography on the next day disclosed no thrombus in either artery and no significant stenosis. Electrocardiographic and laboratory data indicated myocardial infarction. These findings strongly suggest that the combination hormone therapy altered the patient's blood coagulability and played an important role in the formation of the intracoronary thrombi and subsequent acute myocardial infarction.