The effect of granulocyte colony-stimulating factor (G-CSF), given after transplantation, was studied in 155 patients transplanted with haematopoietic stem cells (HSCT) from HLA-identical sibling donors at Huddinge University Hospital between 1993 and 2001. Only patients with haematological malignancies were included. Conditioning consisted of total-body irradiation in 118 and busulphan in 37 patients. They were all given methotrexate combined with cyclosporine as graft-versus-host disease (GVHD) prophylaxis. Of the 155 patients, 66 (43%) received G-CSF after HSCT. Those given G-CSF had a significantly shorter time to neutrophil engraftment (P <0.001). G-CSF treatment had no effect on erythrocyte transfusions, platelet engraftment and infections. However, patients treated with G-CSF had a significantly higher incidence of grades II-IV acute GVHD than those not given this treatment (34 vs 9%, P <0.001). The multivariate analysis showed that the effect of G-CSF was independent of other known risk factors for grades II-IV acute GVHD. Death from GVHD occurred in four and two cases (P=0.06) in the two groups, respectively. The cumulative incidences of transplant-related mortality, survival, chronic GVHD, relapse and relapse-free survival were similar in both groups. In conclusion, G-CSF given after HLA-identical sibling HSCT was associated with a higher risk of grades II-IV acute GVHD, but not transplant-related mortality.