Thionamides, the antithyroid drugs, have been widely used to treat hyperthyroidism1. Side effects are found in 1% - 5% of the patients1. One of the most serious side effect is agranulocytosis, which occurs in 0.1% - 0.3 % of the patients1. This might lead to severe infection and sometimes mortality. The underlying mechanism is unclear. An immune phenomenon may be involved because the antigranulocyte antibodies or lymphocyte sensitized to antithyroid drugs are found in these patients6,7. The recognition of major histocompatibility complex class II peptide complexes by T lymphocytes is central to the development of immune response. According to a report in 1996, the HLA DRB1*08032 allele is strongly associated with susceptibility to methimazole-induced agranulocytosis20.