In infants and children weighing <40kg, it is possible to find a sufficiently matched UCB unit that will deliver a dose of cells critical for successful engraftment (defined as 5 x 10^7 nucleated cells/kg) within a reasonable time frame in >90% of subjects. In teenagers and adults weighing >40kg, this is not always possible. Because UCB units contain a relatively fixed number of total nucleated cells, units delivering optimal cell dosing for subjects weighing >70kg will only be identified <10% of the time. Attempts to increase the dose of cells available for UCBT have included ex vivo expansion and combined unit transplantation. While expansion of UCB cells ex vivo is possible, infusion of these expanded cells have not resulted in shortening of engraftment times. Likewise, combinations of up to 5 UCB units for a single myeloablative transplant have not shortened time to neutrophil or platelet engraftment.