Usually, after double umbilical cord blood transplantation (DUCBT), only 1 of the transplanted units persists in the long term. The characteristics of the winning cord blood unit (W-CBU) that determine unit dominance and how they influence the outcomes of DUCBT remain unclear. We retrospectively analyzed 347 patients with acute leukemia transplanted with a DUCBT (694 CBU) from 2005 to 2013 who had documented neutrophil engraftment and a W-CBU identified by chimerism analysis, to identify unit characteristics impacting on dominance. Median age at DUCBT was 40 years and median follow-up was 35 months. Among W-CBUs, 41% were ≥5/6 HLA matched to the recipient and 59% were ≤4/6. Multivariate analysis indicated that ≤4/6 HLA-matched W-CBUs led to lower leukemia-free survival (44% versus 56%; hazard ratio [HR], 1.5; P = .032) and overall survival (49% versus 62%; HR, 1.5; P = .028), increased nonrelapse mortality (26% versus 18%; HR, 1.9; P = .027), and acute graft-versus-host disease (46% versus 35%; HR, 1.7; P = .013). We were unable to predict unit dominance, but we demonstrated that outcomes were strongly influenced by the degree of HLA mismatch between W-CBU and recipient. Therefore, selection of both units with the lower number of HLA mismatches with the recipient is indicated.