In donor selection for allogeneic stem cell transplant several factors are considered for potential impact on transplant outcome. Previous publications suggested single HLA-mismatched unrelated donors (MMUD) may be equivalent to 10/10 matched unrelated donors (MUDs). We retrospectively examined factors affecting outcome in a single-centre study using ATG followed by post-transplant cyclophosphamide, termed ATG-PTCy, GvHD prophylaxis. Fifty-two patients who received grafts from MMUD and 188 patients transplanted from MUD between January 2015 and December 2019, at Princess Margaret Cancer Centre, Canada, were enrolled. All patients received reduced-intensity conditioning. Overall survival for 9/10 recipients at 2yrs was significantly worse, 37.2% vs 68.5% for 10/10 MUDs, p<0.001, as were NRM at 1yr 39.5% vs 11.7%, p<0.001, and GRFS at 2yrs 29.8% vs 58.8%, p<0.001, respectively, potentially due to higher incidence of infections including CMV. By multivariable analysis factors correlating with survival negatively were DRI, and MMUD, whereas for NRM MMUD and increasing age were unfavourable. For GRFS significant unfavorable factors included donor age≤32y, female donor to male recipient, DRI high-very high and MMUD. These data suggest that MMUD, primarily HLA-A and HLA-B MMUD, confer significantly inferior outcome despite use of ATG-PTCy. Further development of novel conditioning regimens and GvHD prophylaxis is needed to mitigate these risks.