Delayed or failed engraftment remains a concern after cord blood transplantation (CBT) even when using double-unit grafts. Therefore, we analyzed the association between BM assessment performed approximately 21 days after transplantation, and the speed and success of sustained donor-derived neutrophil engraftment in 56 myeloablative double-unit CBT (DCBT) recipients. Overall, the cumulative incidence of sustained neutrophil engraftment was 95% (95% confidence intervals (CI): 89-100). Of the percentage of myeloid precursors, the BM cellularity and the total donor chimerism the total donor chimerism percentage had the most critical association with the speed and success of engraftment. DCBT recipients who were 100% donor achieved a 98% engraftment rate at a median of 22 days. This compared with 100% engraftment in patients who were 90-99% donor, but at a delayed median of 29 days and only 68% engraftment in patients <90% donor at a median of 37 days (P=0.001). Multivariate analysis was performed in the subgroup of patients who had not engrafted at the time the BM analysis was performed, the subgroup of most clinical concern. This confirmed donor chimerism was predictive of subsequent neutrophil recovery (P=0.004). These findings demonstrate the importance of the day 21 BM chimerism determinations after DCBT.