Anti-thyroid drugs are widely used to treat diffuse toxic goiter (Graves' disease). Of the two drugs currently available in the United States, propylthiouracil is prescribed far more often than is methimazole (Tapazole). However, compared with propylthiouracil, methimazole can be given as a single daily dose, is cheaper, and, at low doses, is associated with less major toxicity; for these reasons, methimazole should be used for the routine management of Graves' disease when anti-thyroid drugs are selected as primary therapy. On the other hand, because of certain pharmacologic factors, propylthiouracil should be used in selected situations, particularly in patients with "thyroid storm" and in pregnant or lactating women.