Granulocyte colony-stimulating factor (G-CSF) is widely used to accelerate neutrophil recovery after allogeneic BMT or PBSC transplantation. The optimal time to start G-CSF treatment is not known. Forty-two patients undergoing allogeneic BMT or PBSC transplantation for hematological malignancies received G-CSF either on day 6 or on day 9 post transplant. The time to hematological recovery was monitored and the two groups were compared with respect to peritransplant morbidity and mortality. Recovery of the neutrophil counts to >0.1 x 10(9)/l, > 0.5 x 10(9)/l and >1.0 x 10(9)/l were not significantly different in either group. There was no difference in recovery of red blood cell and platelet counts and no difference between the two groups with respect to the number of febrile days or number of days on antibiotic treatment. Documented bacterial, viral or fungal infections did not occur more often when G-CSF treatment was started on day 9. Delaying treatment with G-CSF resulted in a significant reduction in the length of treatment from 13 to 10 days (23.1% reduction). Reducing the length of the treatment by 3 days lowered the costs by 395.40 Euro per patient. Delaying G-CSF treatment and starting on day 9 after BMT or PBSC transplantation is safe and results in a clear economic benefit.