In recent years, umbilical cord blood (UCB) has widely been used as an alternative hematopoietic stem cell (HSC) source to treat various hematologic disorders in adults. UCB has several advantages over other HSC sources, including rapid availability, and lower or comparable incidence of severe acute graft-versus-host disease even with HLA-mismatched units. However, a high incidence of engraftment failure and early non-relapse mortality preclude UCB from use as a primary HSC source. A unique aspect of UCB transplantation (UCBT) is that Asian countries, particularly Japan, are leading the world in the number of UCBT procedures. This review is intended to give an update of UCBT outcomes in adults and problems to be solved, particularly focusing on engraftment failure.