We investigated the efficacy and safety of high-dose recombinant human granulocyte colony-stimulating factor (rhG-CSF) in treating 10 children with severe aplastic anemia and fewer than 0.05 x 10(9)/L neutrophils. Doses of rhG-CSF ranging from 400 to 2,000 micrograms/m2/d were administered as a 30-minute intravenous infusion daily for 4 weeks. In 6 of the 10 children, treatment increased the neutrophil count by 10- fold to greater than 60-fold (range, 0.21 to 1.8 x 10(9)/L). Bacterial or fungal infections that were present at study entry resolved in all 6 responders, who are still alive with a median survival of more than 27 months (range, 15 to 54 months) since the initiation of treatment. Three of 4 nonresponders died of infection, whereas 1 nonresponder received a bone marrow transplant and is alive. No serious toxicity was attributable to rhG-CSF. It was well tolerated at doses up to 2,000 micrograms/m2/d and effectively stimulated granulopoiesis. This agent thus offers promise as adjuvant treatment for severe infections in children with aplastic anemia and very severe neutropenia.