Serum granulocyte colony-stimulating factor (G-CSF) levels were measured for 24h in 6 patients with hematological disorders and two patients with Graves' disease associated with malignant exophthalmos after iv infusion of high-dose methylprednisolone (HDMP) (20mg/kg BW)for 2h starting at 0900h. Saline solution (500ml) was infused iv for 2h as a control on two days before starting the HDMP therapy. Serum G-CSF levels increased with the mean peak values at 4h after the HDMP therapy (mean±SD: 488.1±125.8pg/ml vs. saline control 74.4±21.9pg/ml, P<0.01). Circulating absolute neutrophil counts (ANC) also increased with the mean peak values at 24h after the HDMP (6, 057±1, 460/μl vs. saline control 2, 007±390/μl, P<0.025). These findings indicate that glucocorticoid has a stimulating effect on G-CSF release and that the increased G-CSF release could be involved, at least partly, in neutrophilia induced by glucocorticoid.