A 26-year-old woman was admitted to hospital with high fever, severe tonsillitis, and gastroenteritis. Because of Graves' disease she had been treated with methimazole for 18 months. Leukopenia and agranulocytosis in combination with a typical bone marrow, exhibiting a complete arrest of myelopoiesis at the stage of promyelocytes led to the diagnosis of an antithyroid therapy induced agranulocytosis. After 1 week of antibiotic treatment without changes in neutrophil counts, granulocyte colony stimulating factor treatment at a dose of 300 micrograms/day subcutaneously was started. Twenty-four hours after the first administration the neutrophil counts began to rise, to 4389/microliters, with a maximum after the third administration and stabilizing at normal levels within 10 days. Since agranulocytosis is considered to be a severe and fatal complication of methimazole therapy, treatment with granulocyte colony stimulating factor seems to be useful for this life-threatening condition.