Three patients, females aged 62, 67 and 32 years, were presented with fever and sore throat and had severe agranulocytosis (granulocyte count < 100/microliter). All had Graves' disease and were being treated with thiamazole 30 mg once a day. Thiamazole was discontinued and treatment with antibiotics initiated. None of the patients received granulocyte-colony stimulating factor (G-CSF). The mean recovery time of granulocytes was 9 days and there were no fatalities. Patients who receive antithyroid agents should be warned against the serious and potentially lethal side effect of agranulocytosis.