Hematopoietic stem cell transplantation (HSCT) is the standard of care in children with Hurler syndrome (HS) as it is the only therapy that can arrest disease progression. We examined the incidence, patterns and outcomes of graft failure in all HS children undergoing first HSCT at the Royal Manchester Children's Hospital or the University of Minnesota Children's Hospital from 1983 to 2016. Implementation of busulfan pharmacokinetic monitoring started in 2004 in both institutions. Two hundred and forty HS children were included in this analysis (historical era (pre-2004), n=131; current era (post 2004), n=109). The proportion of patients with graft failure was significantly lower in the current era compared with the historical era (37.2% vs 10.1%, respectively). Of 49 patients with graft failure in the historical era, 1 had aplasia and 48 had autologous reconstitution. All the 11 graft failures of the current era occurred in recipients of cord blood transplants (7 aplasia and 4 autologous reconstitution). The outcomes of second transplant in these patients has improved, with 89% of such patients alive and engrafted in the current era compared with 58% in the historical era. The pattern of graft failure has changed from autologous reconstitution, likely secondary to inadequate myelosuppression in the historical era, to aplasia in the current era, likely due to imperfect immunosuppression.