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Background : Antithyroid drugs (ATDs) can lead to the development of agranulocytosis, which is the most serious adverse effect. Characteristics of ATD-induced agranulocytosis (AIA) have seldom been reported due to the rarity. In this study, we characterized the clinical features for AIA in Korean patients. Methods: We retrospectively reviewed data from patients with AIA diagnosed between 1997 and 2014 at four tertiary hospitals. Agranulocytosis was defined as an absolute neutrophil count (ANC) below 500/mm³. Results: The mean age of the patients (11 males, 43 females) was 38.2±14.9 years. Forty-eight patients (88.9%) with AIA had fever and sore throat on initial presentation, 20.4% of patients developed AIA during the second course of treatment, and 75.9% of patients suffered AIA within 3 months after initiation of ATD. The patients taking methimazole (n=39) showed lower levels of ANC and more frequent use of granulocyte-macrophage colony-stimulating factor than propylthiouracil (n= 15) users. The median duration of agranulocytosis was 5.5 days (range, 1 to 20). No differences were observed between the long (≥6 days) and short recovery time (≥ 5 days) groups in terms of age, gender, ATDs, duration of ATDs, or initial ANC levels. Four patients (7.4%) who were taking ATDs for less than 2 months died of sepsis on the first or second day of hospitalization. Conclusion: The majority of AIA incidents occur in the early treatment period. Considering the high fatality rate of AIA, an early aggressive therapeutic approach is critical and patients should be well informed regarding the warning symptoms of the disease. [ABSTRACT FROM AUTHOR]