A clinical study to investigate the efficacy and safety of recombinant human G-CSF (rG.CSF) was performed in patients with acute myelogenous leukemia who had had protracted neutropenia. The drug was administered d.i.v. at a dose of 5 micrograms/kg. Sixty-four patients entered the study, of whom 61 patients were evaluable for safety and 58 patients evaluable for efficacy. The treatment produced an early recovery in neutrophil count in the patients who had had protracted neutropenia (< 1,000/microliters) of over 10 days. Among relapsed cases and cases showing > 20% blasts in the bone marrow, many showed blast stimulation in response to rG.CSF, suggesting difficulty in attaining complete remission by subsequent chemotherapy in such cases. The present data indicates that it is desirable to use the drug in lower-blast-count states in order to attain safe and sufficient therapeutic effects in patients with acute myelogenous leukemia who have had protracted neutropenia.