Introduction Excellent survival rates have been reported after allogeneic haematopoietic stem cell transplantation (HSCT) for Wiskott-Aldrich syndrome (WAS) patients. Recipient age >5 years in MUD HSCT as well as MMFD as donor were negative predictors for outcome. However, the vast majority of HSCTs in previously published studies were performed with (oral) busulfan/cyclophosphamide-based conditioning and in the early 2000 years or before. Objectives To compare OS and EFS after HSCT with either busulfan/fludarabine (BuFlu) ± thiotepa (TT) or treosulfan/fludarabine (TreoFlu) ± TT as recommended for primary immunodeficiencies since 2005 by the inborn errors working party (IEWP) of EBMT and ESID. Methods We performed a retrospective analysis via the EBMT and SCETIDE registries of WAS patients transplanted between 20006 and 2016 with these two regimens. At the time of this interim analysis, 174 patients were included, 92 (53%) with BuFlu±TT and 82 (47%) with TreoFlu±TT conditioning, with a median age of 1.6 years (0.2-30) at HSCT and a median follow-up of 32.9 months (1.5-128.9). Donors were MSD in 30, other MRD in 5, MUD (9/10 or 10/10) in 105, MMUD ( Results Two year overall survival (OS) of the entire cohort was 88.6% (95% c.i. 83.5%-93.6%). There was no significant difference in OS between BuFlu±TT or TreoFlu±TT conditioning (2-year OS 88.1% vs. 89.5%; p=0.7). Patients aged >5 years had a worse OS as compared to those 5 years or younger at HSCT (74.9% vs. 90.8%; p=0.005). The type of donor had no influence on OS: 96.4% for MSD/MFD, 86.8% for MUD/MMUD and 87.7% for MMFD (p=0.4). The rate of complete (≥90%) donor chimerism at last follow-up or before a secondary procedure (if a patient had one) was 41/42 (98%) in the BuFlu±TT group and 21/35 (60%) in the TreoFlu±TT group (p=0.0001). Twenty-six patients required a second procedure: stem cell boost in 4, donor lymphocyte infusion in 9, 2nd HSCT in 15 and splenectomy in 1. The 2-year cumulative incidence (CI) of second procedures was higher at 33.9% in the ...