Summary:We performed a case-control analysis of 42 patients with advanced leukemia or MDS comparing peripheral blood stem cell (PBSC) with marrow grafts (BMT) from HLA-matched sibling donors. PBSC were mobilized with G-CSF (7.5 g/kg/day) and yielded a median of 6.7 ؋ 10 6 CD34 ؉ cells/kg (range, 1.6-15.0) and 2.7 ؋ 10 8 CD3 ؉ cells/kg (range, 1.1-7.1) vs marrow grafts with a median of 2.0 ؋ 10 8 nucleated cells/kg (range, 1.8-2.2). Recovery was significantly faster after PBSCT compared to BMT, with a median of 17 (range, 12-26) vs 26 (range, 16-36) days, respectively, to neutrophils Ͼ0.5 ؋ 10 9 /l (P Ͻ 0.01), and 22 (range, 12-Ͼ60) vs 42 (range, 18-Ͼ60) days, for platelet recovery (P Ͻ 0.01). Transplantation of у7 ؋ 10 6 CD34 ؉ cells/kg accelerated recovery to Ͼ20 ؋ 10 9 l platelets; median 17 days (range, 12-19) vs 23 days (range, 17-36) for those receiving Ͻ7 ؋ 10 6 /kg (P = 0.01). PBSC and marrow recipients had similar risks of grades II-IV or III-IV acute GVHD or extensive chronic GVHD (all P Ͼ 0.3). At 1 year after PBSCT and BMT, the risk of relapse was 41% and 32%, respectively (P = 0.47), and the probability of survival was 46% and 48%, respectively (P = 0.70). HLA-matched sibling PBSCT resulted in faster neutrophil and platelet engraftment compared to BMT, with no subsequent differences in acute or chronic GVHD, relapse or survival. A minimum of 7 ؋ 10 6 CD34 ؉ cells/kg in PBSC grafts may be required for very rapid platelet engraftment. Bone Marrow Transplantation (2000) 26, 723-728.