To find a parameter to predict the quality of collected mobilized CD34+ blood as hemopoietic reconstituting cells, the ratio of CFU-GM to CD34+ cells was examined. One hundred six consecutive patients who underwent blood stem cell transplantation at the University of Rochester from 01/01/99 to 12/31/99 were examined retrospectively for the number of days to reach an absolute neutrophil count of 500 or 1000 cells/μl and an absolute platelet count of 20, 000 or 50, 000 cells/μl without transfusion support as measures of engraftment. Linear regression analyses were conducted to determine factors influencing engraftment. The number of CD34+ cells/kg and CFU-GM/kg correlated highly with the number of nucleated blood cells/kg. In this population, in which 90% of patients received >2 × 106 CD34+ cells/kg, neither the number of CD34+ cells/kg nor the number of CFU-GM/kg correlated with the time to engraftment as judged by neutrophil or platelet levels. In contrast, the lower the ratio of CFU-GM to CD34+ cells, the more rapid the reconstitution of platelets to 20, 000/μl (P = 0.03) and 50, 000/μl (P = 0.02). Thus, a lower ratio of the CFU-GM/CD34+ appended to reflect a greater number of hematopoietic reconstituting cells in the blood cell collection. The CFU-GM/CD34+ ratio is an apparent predictor of earlier platelet engraftment, suggesting that the ratio reflects the engraftment potential of mobilized donor progenitor cells.