The number of individuals undergoing unrelated cord blood transplantation (UCBT) has increased in recent years; however, information on prognostic factors is limited. We retrospectively analyzed data from 475 children and adolescents receiving UCBT with myeloablative conditioning for acute lymphoblastic leukemia (ALL) in complete remission (CR), based on a nationwide registry. In the total patient cohort, 5-year leukemia-free survival (LFS) and overall survival (OS) rates after UCBT were 61.1% and 67.7%, respectively. UCBT at first CR and UCBT after 2007 were associated with good survival, while grade II-IV acute graft-versus-host disease (GVHD) was associated with low relapse rate but did not affect survival. Analysis according to human leukocyte antigen (HLA) disparity revealed that tacrolimus-based GVHD prophylaxis resulted in higher OS and lower relapse rate and nonrelapse mortality (NRM) than cyclosporine-based GVHD prophylaxis in patients transplanted with 6/6 and ≤4/6 HLA-matched umbilical cord blood. Furthermore, grade II-IV acute GVHD was associated with good LFS and low relapse rate, without high NRM, in patients receiving 5/6 HLA-matched UCBT. These data indicate that prognostic factors for ALL differ depending on HLA disparity in UCBT.