Myxedema coma is defined as severe hypothyroidism leading to decreased mental status, hypothermia, and other symptoms related to dysfunction in multiple organs. It is very rare disease with high mortality rate. Early recognition and therapy of myxedema coma are essential, and treatment should be begun on the basis of clinical suspection. However, regimen of myxedema is not well established even now, especially about thyroid hormone supplementation. Japan Thyroid Association is drawing up "The diagnostic criteria of myxedema coma (3rd draft) and preliminary guide to treatment of it". According to this criteria and preliminary guide, the clinical presentation, diagnosis, and treatment of myxedema coma will be reviewed here.