The antithyroid agents (carbimazole, methimazole, thiamazole, propylthiouracil and benzylthiouracile) are the drug class that is associated with a high risk of agranulocytosis. Acute and profound (<0.5×10(9)/L) isolated neutropenia occurring in a subject treated with antithyroid agents should be considered as a drug-induced agranulocytosis, until proven otherwise. The clinical spectrum ranges from discovery of acute severe but asymptomatic neutropenia, to isolated fever, localized infections (especially ear, nose and throat, or pulmonary) or septicemia. With an optimal management (discontinuation of antithyroid agents, antibiotics in the presence of fever or a documented infection, or use of hematopoietic growth factor) the current mortality is close to 2%.