Patients with severe angina pectoris and untreated myxedema pose a clinical dilemma because angina may be exacerbated by thyroid hormone replacement therapy, the usual medical management of angina may be ineffective or even dangerous and invasive diagnostic procedures (cardiac catheterization) and surgery have generally been thought to be contraindicated in myxedema. Because of incapacitating progressive angina refractory to medical management, six patients (mean age 58 years) with severe hypothyroidism (mean serum thyroxine level 0.66 μg/100 ml, normal 4 to 11) underwent selective coronary angiography and quantitative biplane left ventriculography. All patients tolerated cardiac catheterization without complication. Five of the six patients had 70 percent or greater stenosis in three coronary vessels, including three patients with left main coronary artery disease. Coronary revascularization was recommended to each patient. One patient refused surgery and subsequently died suddenly. Another patient was scheduled for elective surgery but in the interim died of a massive acute anterior infarction. Four patients underwent saphenous vein bypass grafting to a mean of 2.8 vessels without complication. Preoperative thyroid hormone replacement ranged from 0 to 50 μg of L-thyroxine daily. Postoperatively adequate thyroid hormone replacement was administered to all patients without complication. At follow-up, a mean of 18 months postoperatively, each of the four operative patients is euthyroid, and all have noted either complete resolution or marked amelioration of their angina. It is concluded that (1) patients with incapacitating angina and myxedema often have severe coronary artery disease; (2) cardiac catheterization can be performed safely in the absence of thyroid replacement; (3) coronary revascularization surgery can be performed safely with minimal thyroid replacement; (4) full thyroid replacement can be safely achieved during the postoperative period. © 1977.