Umbilical cord blood transplantation recipients are exposed to an increased risk of graft failure, a complication leading to higher transplantation-related mortality. The decision and timing to offer a second transplant after graft failure is challenging. With the aim of addressing this issue, we analyzed engraftment kinetics and outcomes of 1268 patients (73% children) with acute leukemia (64% acute lymphoblastic leukemia, 36% acute myeloid leukemia) in remission who received single-unit umbilical cord blood transplantation after myeloablative conditioning regimen. Median follow-up was 31 months. Overall survival) at 3-year was 47%; 100-day cumulative incidence of transplant related mortality was 16%. Longer time to engraftment was associated with increased transplant related mortality and lower overall survival. Cumulative incidence of neutrophil engraftment at day-60 was 86%, median time 24 days. Probability density analysis showed that the likelihood of engraftment after umbilical cord blood transplantation increased after day+10, peaked on day+21 and slowly decreased to 21% on day+31. Beyond day+31, the probability of engraftment dropped rapidly, and the residual probability to engraft after day+42 was 5%. Graft failure was reported in 166 patients, and 66 of them received a second graft (allogeneic, n=45). Rescue actions, such as the search for another graft, should be considered starting after day+21; diagnosis of Graft failure can be established for patients not achieving neutrophil recovery by day+42. Moreover, subsequent transplants should not be postponed after day+42.