BACKGROUND The lengthy period of neutropenia which follows allogeneic bone marrow transplantation (BMT) results in significant morbidity and some mortality. Recombinant human granulocyte colony-stimulating factor (rhuG-CSF) effectively reduces neutropenia and morbidity when given after autologous BMT, but has not been adequately investigated in allografts. OBJECTIVE To assess the tolerability, safety and efficacy of rhuG-CSF after allogeneic BMT. METHODS rhuG-CSF was administered to 13 adult patients with haematological malignancies after HLA-identical sibling BMT. Five micrograms/kg of rhuG-CSF was given daily by subcutaneous bolus injection, commencing four hours after marrow infusion and continuing until the neutrophil count was > or = 1.0 x 10(9)/L on three consecutive days. Graft-versus-host disease (GVHD) prophylaxis was cyclosporin and short-course methotrexate (days 1, 3, 6 and 11). Prophylactic intravenous (IV) antibiotics were administered from the onset of neutropenia. The control group consisted of patients with comparable diagnoses, transplanted before and after the current study using identical supportive care and GVHD prophylaxis policies. RESULTS Although time to recovery of the neutrophil count to > 0.1 x 10(9)/L was similar, the rhuG-CSF-treated patients experienced accelerated recovery to > 0.5 x 10(9)/L, which occurred at a median of 15 days (range 11-21) after marrow infusion in study patients compared to 18.5 days (range 14-41) in the controls (p = 0.04). No significant differences were detected in any of the indices of transplant-related morbidity examined, including the number of days of fever, the incidence of culture-positive infections, the usage of antibiotics, the requirement for parenteral nutrition and IV morphine, the maximum severity of mucositis and GVHD, and the day of discharge. CONCLUSIONS Within the context of this study, rhuG-CSF had limited impact on the clinical outcome of HLA-identical sibling BMT.