Allogeneic mobilized peripheral blood progenitor cells instead of bone marrow are increasingly used to restore hematopoiesis after myeloablative therapy. Data supporting this important change of clinical practice are scarce. We therefore assigned patients with early leukemias to peripheral blood or bone marrow transplantation; the occurrence of acute and chronic graft versus host disease, survival, transplantation-related mortality, and relapse rates were compared. A total of 350 patients between 18 and 55 years of age with acute leukemias in remission or chronic myelogenous leukemia in first chronic phase were randomized to receive either filgrastim-mobilized peripheral blood progenitor cells or bone marrow cells from HLA-identical sibling donors after standard high-dose chemoradiotherapy. Neutrophil and platelet recovery occurred significantly faster after transplantation of peripheral blood progenitor cells than after bone marrow transplantation. Acute graft versus host disease of grades II-IV was significantly more frequent in recipients of peripheral blood progenitor cells than in recipients of marrow cells (52% vs 39%, odds ratio 1.74, 95% confidence interval 1.12-2.69, P =.013). The cumulative incidence of chronic graft versus host disease was 67% with peripheral blood progenitor cells and 54% with bone marrow cells (hazard ratio 1.67, 95% confidence interval 1.15-2.42, P =.0066). The estimated overall probability of survival at 2 years was 65% with either source of stem cells (hazard ratio 1.15, 95% confidence interval 0.79-1.67, P =.46). Disease-free survival, transplantation-related mortality at day 100, and relapse rates did not significantly differ between treatment arms. Peripheral blood is an equivalent source of hematopoietic stem cells compared with bone marrow if administered to patients with standard-risk leukemias. Long-term observation of patients with different diseases and stages of disease is necessary to ultimately define the role of both sources of stem cells.