Myxedema coma is a medical emergency which requires prompt action to avoid an almost uniformly fatal outcome. The condition may begin insidiously when an elderly patient, who has been hypothyroid for a long time, lapses into coma, or it may occur dramatically in mildly hypothyroid people following therapy with tranquilizers, narcotics, or sedatives. Anesthesia or infections may also precipitate the coma. Usually CO2retention, somnolence, hyponatremia, bradycardia, hypotension, or hypothermia precede the onset of the coma.Prevention of coma consists of lifelong maintenance of thyroid hormone replacement therapy in the hypothyroid subject. In the untreated hypothyroid subject, one must heed those conditions which presage the onset of coma and institute thyroid replacement promptly. Once coma has occurred, therapy consists of the rapid replacement of large amounts of thyroid hormone and of supportive measures to counteract the complications of coma. The management of CO2retention by establishing an adequate airway and the administration of adrenal steroids are important adjuncts to therapy.