We report the outcome of 236 pediatric umbilical cord blood transplantations (UCBT) performed in Korea. Given that the sources of the grafts were mostly unrelated donors (n = 226; 95.8%), only the results of unrelated UCBT were included for all statistics. The most frequent primary disease was acute leukemia (n = 167). In total, 91.7% of recipients were seropositive for cytomegalovirus (CMV). The median doses of nucleated cells and CD34+ cells were 4.84 × 107/kg and 2.00 × 105/kg, respectively. The median times to neutrophil (>0.5 × 109/L) and platelet recovery (>20 × 109/L) were 18 and 45 days, respectively. Grade 2–4 acute graft-versus-host-disease (GVHD) and chronic GVHD developed in 41.1 and 36.1% of cases, respectively. Forty-five patients developed CMV disease. The 5-year overall and event-free survival were 47.5 and 36.9%, respectively. Multivariate analysis revealed that adverse factors for survival of the whole cohort were total body irradiation-based conditioning (P = 0.007), salvage transplant (P = 0.001), failure to achieve early complete chimerism (P < 0.0005), and CMV disease (P = 0.001). The outcomes of the single- and double-unit UCBT (n = 64) were similar, while double-unit recipients were heavier (P < 0.0005) and older (P < 0.0005). We conclude that double-unit UCBT is a reasonable option for older or heavier children and that the thorough surveillance of CMV infection and the development of an effective CMV therapeutic strategy may be especially important for Korean children, whose CMV seroprevalence exceeds 90%. Am. J. Hematol., 2011. © 2010 Wiley-Liss, Inc.