This prospective multicenter study examined whether simultaneous administration of granulocyte colony-stimulating factor (G-CSF; Filgrastim) and induction chemotherapy for adult acute lymphoblastic leukemia (ALL) could prevent treatment-related neutropenia, infections, and resulting treatment delays. Seventy-six patients were randomly assigned to receive either G-CSF (n = 37) or no growth factor (n = 39) in conjunction with a uniform chemotherapy consisting of cyclophosphamide, cytarabine, mercaptopurine, intrathecal methotrexate, and cranial irradiation. The median duration of neutropenia (absolute neutrophil count < 1 x 10(9)/L) during chemotherapy was 8 days in patients receiving C-CSF, compared with 12.5 days in the control group (P < .002). A similar reduction from 11.5 to 7 days was observed in patients with T-ALL receiving additional mediastinal irradiation (P = .13). Infections occurred in 43% and 56% of patients in the G-CSF and control arm, respectively (P = .25); the incidence of nonviral infections was reduced by 50%, from 32 episodes in the control arm to 16 episodes in the G-CSF arm. Prolonged interruptions of chemotherapy administration were less frequent, with delays of 2 weeks or more occurring in only 24% of patients receiving G-CSF as opposed to 46% in the control arm (P = .01). Accordingly, chemotherapy was completed significantly earlier with the use of G-CSF (39 v 44 days, P = .008). With a median follow-up of 20 months, the probability of disease-free survival was 0.45 in the G-CSF group and 0.43 in the control group (P = .34). In conclusion, adult ALL patients appear to benefit by the simultaneous administration of G-CSF with induction chemotherapy because of a significant reduction in the duration of neutropenia, a trend to fewer infections, and a more rapid completion of chemotherapy.