To assess the influence of graft-versus-host disease (GVHD) on the outcome of patients with advanced haematological malignancies (AHM) who received a primary, unmodified allogeneic peripheral blood progenitor cells transplant (allo-PBT) from a human leucocyte antigen (HLA) identical sibling donor, we analysed 136 patients with myeloid neoplasms (n = 70) or lymphoproliferative disorders (n = 66), transplanted at 19 Spanish institutions. Median age was 35 years (range 1-61). The cumulative incidence of relapse for all patients was 34% (95% CI, 26-42%), 41% (95% CI, 33-49) for patients without GVHD and 14% (95% CI, 3-25) (P = 0.001) for patients with acute and chronic GVHD. After a median follow-up of 11 months (range 2-49), 60 (44%) patients remained alive with an actuarial probability of overall survival and disease-free survival (DFS) at 30 months of 31% (95% CI, 21-41%) and 28% (95% CI, 17-39%) respectively. In patients surviving > 100 d, the low incidence of relapse in those with acute and chronic GVHD led to a DFS of 57% (95% CI, 38-76%) compared with a DFS of 34% (95% CI, 17-51%) in the remaining patients (P = 0.03). Our results indicate a reduced incidence of relapse for patients with AHM receiving an unmodified allo-PBT and developing acute and chronic GVHD, which results in an improved DFS.