A study was done to compare treatment with Filgrastim (r-metHuG-CSF) given at three different times after unrelated bone marrow transplantation (BMT). Sixty-nine patients grafted with HLA-A, -B and -DR-compatible unrelated bone marrow were randomized to Filgrastim (5 microg/kg/day) starting on day 0 (n = 23), day +5 (n = 23) or day +10 (n = 23) after BMT. No significant differences were detected in hematological recovery, days with fever, days on antibiotics, incidence of bacteremia or need for erythrocyte, platelet and granulocyte transfusions between the three groups. Patients given Filgrastim starting on day 0, day +5 or day +10, respectively, reached an absolute neutrophil count (ANC) >0.5 x 109/l on a median of 17, 16 and 16 days after BMT. Starting Filgrastim treatment on day +10, rather than on day 0, reduced the costs of Filgrastim by $1060, with no significant change in the median number of days-to-hospital discharge in the three Filgrastim-treated groups. The incidences of acute and chronic GVHD, transplantation-related mortality, relapse, leukemia-free survival and patient survival (PS) were similar in all groups.