Antithyroid drugs are widely used to treat hyperthyroidism, especially Graves' disease, but they tend to cause agranulocytosis, which increases the mortality rate. Granulocyte colony-stimulating factor decreases the duration of recovery from agranulocytosis. We retrospectively studied cases of antithyroid drug-induced agranulocytosis over the past 10 years in a northern Taiwan medical center. A clinical evaluation was conducted, including a review of complete blood cell counts and differential counts. Four cases were included in this analysis. Agranulocytosis persisted in 2 cases despite a change in therapy from propylthiouracil to methimazole. Fever, sore throat, and diarrhea were common symptoms of agranulocytosis. Initial white blood cell counts ranged from 450 to 1,710/microL. Only 1 case had a positive result from a throat swab culture (Staphylococcus aureus). Three of 4 cases received granulocyte colony-stimulating factor therapy, and the recovery time ranged from 3 to 13 days. All of the patients recovered from agranulocytosis. We concluded that: (1) conducting a routine complete blood cell count is beneficial in alerting caregivers to the possibility of agranulocytosis; (2) educating patients about the common symptoms of agranulocytosis may contribute to an early diagnosis; (3) providing granulocyte colony-stimulating factor therapy to patients results in good prognosis; and (4) monitoring for cross-reactions between drugs should be performed to prevent further episodes of agranulocytosis.