Seventy-eight patients with haematological malignancies, received T-cell-depleted stem cell transplants and cyclosporin followed by delayed add-back of donor lymphocytes to prevent leukaemia relapse. The source of stem cells was bone marrow in 50 patients and granulocyte colony-stimulating factor (G-CSF)-mobilized peripheral blood in 28 patients. In univariate analysis, only the CD34+ cell dose (but not the stem cell source or the T lymphocyte dose) and disease status were predictive for transplant-related mortality, relapse and survival. Patients receiving ≥ 3 × 106 CD34+ cells/kg had an overall actuarial survival of 68% compared with 52%, 35% and 10%, respectively, for cell doses of 2–2.99, 1–1.99 and < 1 × 106/kg. Multivariate analysis of risk factors for relapse identified disease risk and CD34+ cell dose as the only factors. Relapse was 62.5% in 38 patients at high risk of relapse vs. 25% for 40 patients at intermediate or low risk. CD34+ cell doses of ≥ 3 × 106/kg were associated with a 13.5% relapse vs. 48% for recipients of lower doses. This favourable effect of CD34+ cell dose on relapse was apparent in both high- and intermediate- plus low-risk groups. Our results support the potential benefit of a high stem cell dose in lowering transplant-related mortality (TRM) and in reducing relapse after allogeneic marrow or blood stem cell transplants.