We have conducted a retrospective study on 251 patients from three centers in France and Switzerland between 2004 and 2010 with the goal to evaluate the impact of HLA-DRB3/B4/B5 allele mismatching after HLA-10/10-matched unrelated allogeneic hematopoietic stem cell transplantation (HSCT). Fourteen (5.5%) patients receiving HSCT from an HLA-10/10-matched unrelated donor had a mismatched DRB4 donor, 23 (9.5%) patients had a mismatched DRB3 donor and 214 (85%) had a fully matched unrelated donor (HLA-10/10) without DRB3- or DRB4-mismatched donor. We compared the outcomes of 37 patients with a DRB3 or DRB4 mismatch with the rest of the population. The median survival for a patient without DRB3/4 mismatch was 18 months (95% confidence interval (CI), 13-29), for DRB3-mismatched patients 32 months (95% CI, 13-NR) and for DRB4-mismatched patients 7 months (95% CI, 3-NR). The multivariate analysis showed a significant impact of DRB4 mismatching on survival (Hazards ratio (HR)=2.1 (95% CI, 1.01-4.67), P=0.045), acute GvHD (HR=2.66 (95% CI, 0.99-7.09) P=0.05) and on transplant-related mortality (HR=2.8; (95% CI, 1.7-4.4) P=0.024). In the view of an impact of DRB4 locus mismatch on clinical outcome, it would be important to confirm this observation in a prospective study as it may be worth considering DRB4 in the unrelated donor selection.