Methimazole has remained the cornerstone for the treatment of hyperthyroidism since 1940's and it is a well-tolerated antithyroid drug. Pancytopenia is one of the major side effects of methimazole, but its occurrence is very rare. There have been some case reports about methimazole-induced pancytopenia that was treated with recombinant human granulocyte colony-stimulating factor (G-CSF), but its usefulness is still controversial. We present here a case of a 50-year-old female who had been treated with methimazole for hyperthyroidism and she subsequently presented pancytopenia. G-CSF was given for 10 days and she successfully recovered from the pancytopenia.