Purpose: To determine the number of CD34+ cells associated with a high probability of rapid engraftment after allogeneic peripheral-blood stem-cell (PBSC) transplant, and to examine the relationship between certain donor characteristics and the effectiveness of PBSC mobilization. Patients and Methods: Between December 1994 and July 1996, we treated 47 patients who had resistant hematologic neoplasms with myeloablative therapy followed by transplantation of allogeneic PBSC collected from histocompatible siblings after mobilization with granulocyte colony-stimulating factor (G-CSF). Expression of CD34 was determined by flow cytometry. Results: Engraftment was rapid and similar to that observed following autologous PBSC transplant, with an absolute neutrophil count (ANC) greater than 500/μL and platelet count greater than 20,000/μL on median days +9 and + 11, respectively. The pace of hematologic recovery correlated with the number of hematopoietic progenitors transplanted, so that patients who received greater than 5 x 106 CD34+ cells/kg recipient weight had a 95% likelihood of neutrophil and platelet recovery by day + 15. Baseline (precytokine) CD34+ cells per milliliter of donor peripheral blood and total G-CSF dose (donor weight x 10 μg/kg) correlated with the number of CD34+ cells collected (R2 = .24 and P = .0009, and R2 = .24 and P < .0001, respectively). Donor age and sex did not effect mobilization. Conclusion: Following allogeneic PBSC transplant, patients who received greater than 5 x 106 CD34+ cells/kg recipient weight had a high probability of rapid engraftment. Donors with low baseline levels of circulating progenitors (< 2,000 CD34+ cells/mL blood) and those who received lower total doses of G-CSF were less likely to be effectively mobilized. For donors with low baseline CD34+ counts, higher doses of G-CSF might improve mobilization. Baseline CD34+ counts and total G-CSF dose accounted for less than half of the variation in CD34+ cells collected, which indicates that other, as yet unidentified, factors play an important role in determining the effectiveness of mobilization.