To determine the impact of high-resolution (HR) HLA typing with outcomes after UCBT, DNAs of 122 pairs were analysed for HLA class I and class II mismatches (MM) based on HR typing. For HLA-A, -B on low-resolution typing and -DRB1 on HR typing, the following MM situation resulted: no MM (13%), one MM (40%), two MM (36%), three MM (8%), four MM (3%). For A, B, C, DR and DQ based on HR typing the following MM occurred: No MM (4%), one MM (10%), two MM (15%), three MM (22%), four MM (25%), five MM (12%), six MM (6%), seven MM (3%), eight MM (2%). There was no significant association between number of MM (HR) for both HLA-A, -B and -DRB1 and HLA-A, -B, -C, -DRB1 and DQB1 and aGvHD grade II-IV. There was a trend that MM in class I HR were associated with neutrophil recovery; HLA-A locus typing analysed in HvG direction was associated with reduced cumulative incidence of engraftment (P=0.04), the same for C-KIR in HvG direction (P=0.01). No significant correlation was found between numbers of HLA-MM on the HR level with 2-year survival. The analysis shows that the degree of mismatching in UCBT is even higher than expected.