A total of 1634 recipients of HLA-identical sibling bone marrow with acute leukemia were treated with the combination of cyclosporin A (CsA) and methotrexate as prophylaxis against graft-versus-host disease (GVHD). The probability of relapse decreased with an increasing grade of acute GVHD, especially in patients grafted in first remission (CR-1): P < 0.01 and P < 0.001 for acute lymphoblastic leukemia and acute myeloid leukemia, respectively. Among patients surviving at least 3 months without a relapse, chronic GVHD was associated with a decreased incidence of relapse in CR-1 (P < 0.0001 for both diagnoses), and a better LFS. Among patients in CR-1, the probability of relapse was the same in those with limited or with extensive chronic GVHD. However, in patients with intermediate stage of the disease (> or = 2nd remission or 1st relapse) with previous acute GVHD, those with extensive chronic GVHD had a reduced probability of relapse (P = 0.05). The graft-versus-leukemia effect of acute and especially chronic GVHD was confirmed by multivariate analyses. Overall, the highest LFS was seen in patients with chronic GVHD and no or grades I-II acute GVHD. The lowest LFS was seen in patients having acute GVHD grades III-IV without chronic GVHD.