The aim of this study was to evaluate the feasibility of CMF 1.8-28 regimen, with all three drugs administered intravenously (IV), and to compare the hematologic toxicity of this regimen with or without G-CSF. Patients aged 18 to 65 years with histologically proven breast cancer treated by surgery and without distant metastases were eligible for the study. All patients had to have normal white blood cells (WBC) count (WBC > or = 3000/mm3 and/or neutrophils > or = 2000), and platelets (Plt) counts (> or = 100,000/mm3), and adequate renal and hepatic function. The toxicity was recorded according to World Health Organization Scale. CMF 1.8 regimen was: cyclophosphamide 600 mg/m2, methotrexate 40 mg/m2, 5-fluorouracil 600 mg/m2. The drugs were given IV on day 1 and 8, and cycles repeated every 28 days. G-CSF (5 micrograms/kg/day) was administered subcutaneously from day 9 to 20, starting from the second cycle of chemotherapy. A complete blood count with white-cell differential and platelet count was obtained twice a week. For each patient bone marrow toxicity variables recorded during the first cycle (without G-CSF) were compared with values during the second cycle (with G-CSF). One of 10 entered patients, 1 was not evaluated due to missing data on hematologic toxicity. All patients received chemotherapy with or without G-CSF at the scheduled 28th day. Treatment with G-CSF after CMF 1.8 regimen resulted in a significantly WBC's earlier nadir (average day of nadir 14 vs 17; p = 0.0005), while there was no difference in the average values of WBC at the nadir. Moreover, the average value of platelets recorded at the nadir was significantly lower with the use of G-CSF (average No. of platelets/mm3; 185,111 vs 116,000; p = 0.001). Complete hematologic recovery without and with G-CSF was reached on day 25 and 20 respectively (p = 0.001). CMF 1.8 with IV cyclophosphamide is a feasible regimen with and without G-CSF and can be used in adjuvant setting instead of "classic" CMF in order to improve the low compliance observed when cyclophosphamide is given by mouth. As reported by others, we observed that after standard chemotherapy G-CSF anticipated the nadir of WBC and hastened hematologic recovery (WBC > 3000 and Plt > or = 100,000).(ABSTRACT TRUNCATED AT 400 WORDS)