This study examined whether granulocyte colony-stimulating factor (G-CSF) is beneficial for the treatment of antithyroid drug-induced agranulocytosis. From January 1975 to December 2001, 30,798 patients with Graves' disease were treated with antithyroid drugs at Noguchi Thyroid Clinic & Hospital Foundation. During this period, 109 patients (0.35%) were found to have agranulocytosis caused by antithyroid drugs. In the symptomatic group, the recovery time from agranulocytosis was significantly shorter after the introduction of G-CSF (5.5 +/- 3.5 days, n = 19) compared to the symptomatic group before its introduction (9.2 +/- 4.4 days, n = 37, p < 0.01). In the asymptomatic group, the recovery time from agranulocytosis was significantly shorter after the introduction of G-CSF (2.3 +/- 1.9 days, n = 15) compared to the asymptomatic group before the introduction of GCSF (5.4 +/- 4.3 days, n = 34, p < 0.05). However, G-CSF therapy was ineffective in severe cases with granulocyte count below 0.1 x 10(9)/L and symptoms. We recommend that G-CSF therapy should be applied only in asymptomatic patients and symptomatic patients with granulocyte count above 0.1 x 10(9)/L, and not for symptomatic patients with granulocyte count below 0.1 x 10(9)/L. In conclusion, G-CSF therapy shortens the period of recovery from antithyroid drug-induced agranulocytosis and benefits patients, except those with symptoms and a granulocyte count below 0.1 x 10(9)/L.