The purpose of the present study was to evaluate the relation between the grade of acute graft-versus-host disease (GVHD) and several outcome parameters. Special attention was given to leukaemia-free survival (LFS). The study included 2,122 patients with acute leukaemia in first remission and 780 patients with chronic myeloid leukaemia (CML) in the first chronic phase, who were receiving first transplants from HLA-identical donors. Transplant-related mortality (TRM) was the same in patients with or without grade I acute GVHD, but was increased in patients with grades II, III and IV acute GVHD. In patients with grades 0, I, II, III and IV acute GVHD, the five-year probability of TRM (+/-SE) overall was 20 +/- 1%, 19 +/- 2%, 30 +/- 2%, 61 +/- 4% and 90 +/- 3%, respectively. The incidence of relapse in patients with grades I-IV was lower than in patients without acute GVHD. The five-year relapse probability according to GVHD grade (0-IV), was 28 +/- 2%, 22 +/- 2%, 22 +/- 2%, 17 +/- 5% and 20 +/- 2%. The five-year LFS in all patients with leukaemia, according to grade of acute GVHD, was 57 +/- 2%, 63 +/- 2%, 55 +/- 3%, 32 +/- 4% and 8 +/- 3%. In multivariate analysis, adjustments for age, sex, diagnosis, interval from diagnosis to transplantation, T-cell depletion and year of transplantation were considered. Patients with grade I acute GVHD had the highest LFS (relative risk 0.84, p = 0.04 compared with those without acute GVHD).