Propylthiouracil and methimazole are used widely in the treatment of hyperthyroid disorders. The most important complication of the use of these drugs is depression of the neutrophilic granulocyte count. Granulocytopenia occurs in about 4 percent and agranulocytosis occurs in about 0.3 percent of treated patients. Although this depression of the granulocyte count is reversible after the drug is discontinued, serious infection frequently accompanies agranulocytosis and accounts for almost all deaths related to the drugs. It is important to be aware of the clinical features of granulocytopenic reactions due to antithyroid drugs.