Radiotherapy patients are at risk of developing leukopenia, which risk depends on the irradiated volume, the rate of irradiated bone marrow and the radiation dose. Radiogenic leukopenia may cause radiotherapy drop-out, with consequent effects, on local tumor control and clinical outcome. The introduction of granulocyte growth factors, such as filgrastim, has permitted to accelerate normal neutrophil count recovery in irradiation-related neutropenia both in vitro and animal models; clinical experience in humans is still lacking, relative to both indications and scheduling. In the Oncologic Radiotherapy Department of Treviso Hospital, 31 patients irradiated for Hodgkin disease, rectal cancer and other malignancies, who presented leukopenia requiring treatment discontinuation, were given filgrastim to assess its actual effect in avoiding further drop-outs and to compare two administration schedules (2 or 3 vials, 30 MIU, weekly). Filgrastim treatment was continued throughout the radiotherapy cycles, for 1 to 5 weeks. Eighteen patients had received previous chemotherapy and 11 were undergoing concurrent 5-fluorouracil chemotherapy-irradiation. A mean 203% increase in leukocyte count was observed (136% in the patients treated with 2 vials/week and 274% in those receiving 3 vials/week); this increase was more apparent in women that in men (256% versus 91%) and slightly higher in patients 50 years old and with target volumes < 5000 ml. Filgrastin treatment was well tolerated by all patients, with no discontinuations due to adverse effects; 9 patients (29%) reported skeletal pain, which was marked in 2 of them only. Eighty percent of patients completed all the radiotherapy cycles with no discontinuation, while 6 patients dropped out because leukopenia persisted. Biweekly filgrastim administration was effective to prevent unscheduled radiotherapy discontinuation in 75% of patients and triweekly administration was effective in 86% of patients. In our experience, filgrastim administration was well tolerated and effective in decreasing the irradiation drop-outs caused by treatment-related leukopenia. Since this drug is rather expensive, we decided to use routinely the lower dosage of biweekly administration (with one vial given on Friday and Saturday, to permit neutrophil recovery during the day off) and to reserve the higher dosage (3 vials a week) to the patients with large body areas, big target volumes and persistent leukopenia during previous chemotherapy.