Ten patients given HLA-identical sibling marrow transplants for lymphoid malignancy received recombinant human granulocyte macrophage-colony stimulating factor (GM-CSF) from day 7 to day 13 inclusive post transplant. Patients were prepared for transplantation with busulphan 16 mg/kg and cyclophosphamide 120 mg/kg. Immunosuppression to minimise the risk of graft-versus-host disease (GVHD) was cyclosporin/short methotrexate. Results were compared with a historical control group of patients (n = 16) given matched sibling transplants for acute leukaemia and receiving the same immune suppressive regime but not given GM-CSF. Recovery of total white cells, neutrophils, monocytes and lymphocytes was more rapid in the GM-CSF recipients (p less than 0.02). There was a suggestion of a decrease in non-viral infections in the first 30 days in the GM-CSF recipients (p = 0.09). There was, however, no significant difference in the severity of oropharyngeal mucositis nor in the duration of the transplant hospitalisation. Surprisingly, the severity of acute GVHD was higher in the GM-CSF recipients with six of eight evaluable patients having grade II-IV acute GVHD (p = 0.003). Two GM-CSF recipients developed a fluid retention/capillary leak syndrome. These findings indicate a need for caution in the use of GM-CSF after allogeneic marrow transplantation.