We evaluated the effects of recombinant human granulocyte colony stimulating factor (rhG-CSF) given to 30 patients with hematological malignancies after cytotoxic chemotherapy. The first course of chemotherapy was not treated with rhG-CSF (control), and in the second to fourth courses, rhG-CSF was given by one of the following three ways to the patients (not necessarily in this order): 1) 10 days of administration starting 48 hr after chemotherapy, 2) 5 days of administration starting 48 hr after chemotherapy, and 3) 5 days of administration after the leukocyte counts reached to less than 2,000/microliters. The leukocyte nadirs were significantly higher in the course with 10 days of administration compared with the control course. The time needed for recovery from the leukocyte nadir was significantly shorter in 10-day course and 5-day course after the leukocyte counts reached to less than 2,000/microliters. The therapy spans became significantly shorter with all of the three patterns of administration of rhG-CSF compared with the control course. The number of days on which the leukocyte counts became less than 2,000/microliters were significantly fewer in 10-day course and 5-day course after the leukocyte counts became less than 2,000/microliters. These findings showed that rhG-CSF prevented severe neutropenia after cytotoxic chemotherapy, and (or) assisted the rapid recovery from neutropenia. These effects depend on the timing of its administration.