In a randomised, double-blind trial 20 patients with leukaemia received human recombinant granulocyte macrophage colony-stimulating factor (GM-CSF) and 20 received placebo, for 14 days after allogeneic, matched sibling, bone-marrow transplantation. The neutrophil count recovered to 0.5 x 10(9)/l 3 days earlier in the GM-CSF group than in the placebo group (not significant), and the median neutrophil count at 14 days was significantly higher in the GM-CSF group (1.90 vs 0.46 x 10(9)/l). The lymphocyte count was significantly higher in the GM-CSF than in the placebo group between days 10 and 15 after transplantation, but this difference was not associated with a higher incidence of graft-versus-host disease. There was no evidence that GM-CSF was associated with a greater incidence of leukaemic relapse. The GM-CSF group had lower haemoglobin concentrations and platelet counts and higher plasma urea, creatinine, and bilirubin than the placebo group. The duration of hospital stay was the same for both patient groups. Further studies are now indicated to assess the overall effect of GM-CSF on outcome after allogeneic bone-marrow transplantation.