The effect of granulocyte colony-stimulating factor (G-CSF) on neutropenia and infection was studied in a randomized trial in patients receiving intensive chemotherapy for acute leukemia. Fifty seven patients with acute leukemia (35 cases of refractory acute myeloid leukemia, 19 cases of acute lymphoblastic leukemia and 3 cases of blast crisis of chronic myeloid leukemia) were given G-CSF under either of the following two conditions; 1) Group A: starting G-CSF (200 micrograms/m2iv) administration 24 hrs after chemotherapy. 2) Group B: the same dose of G-CSF administration after a febrile episode of 38 degrees C with neutropenia (less than 1,000/microliters). Five patients were excluded from the study. Group A (27 patients) showed a shorter febrile period (2.15 +/- 2.98 days) than the 25 patients of Group B (3.40 +/- 4.78 days), but the difference was not statistically significant. Compared to Group B, Group A showed significantly early recovery of neutrophil counts as well as early recovery from documented infections. There was no evidence that early administration of G-CSF accelerates the growth of leukemic cells nor causes early relapse of acute leukemia.