PURPOSE: This was the first randomized study to investigate the efficacy of peripheral-blood progenitor cell (PBPC) mobilization using stem-cell factor (SCF) in combination with filgrastim (G-CSF) following chemotherapy compared with filgrastim alone following chemotherapy. PATIENTS AND METHODS: Forty-eight patients with ovarian cancer were treated with cyclophosphamide and randomized to receive filgrastim 5 microg/kg alone or filgrastim 5 microg/kg plus SCF. The dose of SCF was cohort-dependent (5, 10, 15, and 20 microg/kg), with 12 patients in each cohort, nine of whom received SCF plus filgrastim and the remaining three patients who received filgrastim alone. On recovery from the WBC nadir, patients underwent a single apheresis. RESULTS: SCF in combination with filgrastim following chemotherapy enhanced the mobilization of progenitor cells compared with that produced by filgrastim alone following chemotherapy. This enhancement was dose-dependent for colony-forming unit-granulocyte-macrophage (CFU-GM), burst-forming unit-erythrocyte (BFU-E), and CD34+ cells in both the peripheral blood and apheresis product. In the apheresis product, threefold to fivefold increases in median CD34+ and progenitor cell yields were obtained in patients treated with SCF 20 microg/kg plus filgrastim compared with yields obtained in patients treated with filgrastim alone. Peripheral blood values of CFU-GM, BFU-E, and CD34+ cells per milliliter remained above defined threshold levels longer with higher doses of SCF. The higher doses of SCF offer a greater window of opportunity in which to perform the apheresis to achieve high yields. CONCLUSION: SCF (15 or 20 microg/kg) in combination with filgrastim following chemotherapy is an effective way of increasing progenitor cell yields compared with filgrastim alone following chemotherapy.