We retrospectively analysed outcomes among 395 patients with haematologic malignancies who underwent non-myeloablative haematopoietic cell transplantation (HCT) from human leucocyte antigen (HLA)-matched related (n = 297) or unrelated donors (n = 98) in order to identify a possible correlation between the time of onset of graft-versus-host disease (GVHD) and survival. The non-myeloablative regimen consisted of 2 Gy total body irradiation with or without fludarabine, followed by postgrafting immunosuppression with mycophenolate mofetil and cyclosporine. The cumulative incidences of grades II-IV acute GVHD and extensive chronic GVHD were 45% and 47%, respectively, with related donors, and 68% and 68%, respectively, with unrelated donors. High-dose corticosteroid treatment for acute or chronic GVHD was started at a median of 79 (range, 8-799) days and 30 (range, 5-333) days after transplantation from related and unrelated donors respectively. With related donors, the cumulative incidence of non-relapse mortality among patients with GVHD was 55% at 4 years when prednisone was started before day 50 (n = 72), compared with 29% when treatment was started after day 50 (n = 115) (P < 0.001). With unrelated donors, time to onset of treatment for GVHD was not associated with survival. Patients with early-onset GVHD after non-myeloablative HCT from HLA-identical related donors might benefit from intensified primary immunosuppressive treatment.