A randomised trial in breast cancer patients was designed to compare the number of peripheral blood progenitor cells collected after mobilisation with a single dose of 10 microg/kg/day granulocyte colony-stimulating factor (G-CSF) (n=14) or a split dose of 5 microg/kg twice daily (n=14). Both groups were well balanced. No significant differences were observed between groups regarding aphereses parameters. The total number of CD34+ cells collected was higher in the split-dose group (mean of 7.1 and median of 7.4 x 10(6)/kg) than in the single-dose group (5.6 and 5.8 x 10(6)/kg, respectively) (P=0.26). The mean of CD34+ cells collected after the first apheresis procedure was 3.9 x 10(6)/kg for the split dose group and 3.1 x 10(6)/kg for the single-dose group (P=0.24). Circulating CD34+ cells before the first apheresis were higher for the split-dose group (mean 79.7 vs 59.2 x 10(6)/l) (P=0.14). All bone pain scores applied were significantly higher for the split-dose group. Our primary end point of improving the mean of total CD34+ cells collected to 2.5 x 10(6)/kg was not achieved with twice-daily G-CSF administration. Further studies evaluating different mobilisation schedules with G-CSF are needed to determine the optimal regimen.