Background/Aim: Comparison of transplant outcomes in long-term follow-up of children after total body irradiation (TBI)-or chemotherapy-based conditioning allogeneic hematopoietic cell transplantation (allo-HCT).Patients and Methods: Patients undergoing allo-HCT for Acute lymphoblastic leukemia (ALL) conditioned either with TBI (n=55) or chemotherapy (n=84) were compared. The following transplant outcomes were analyzed: overall survival (OS), event-free survival (EFS), relapse incidence (RI), and graftversus-host-disease (GVHD)-free-relapse-free survival (GRFS). Results: All analyzed long-term transplant outcomes were significantly better for patients conditioned with TBI at 2 years after transplant. OS at 2 years was 84% after TBI and 60.5% after chemotherapy-conditioning (p=0.005). Risk factor analysis showed that two factors, TBI-based conditioning and transplant in first remission of ALL, significantly improved OS, EFS, GRFS, and decreased RI. Conclusion: TBI-based conditioning before allogeneic HCT in children with acute lymphoblastic leukemia provides significantly better transplant outcomes, when compared to chemotherapy-based conditioning.Acute lymphoblastic leukemia (ALL) is the most frequent type of malignancy in children. With international cooperation, outcomes have improved remarkably during the last decades and reached 90% of long-term survival (1, 2). ALL is also the most frequent indication for allogeneic hematopoietic cell transplantation (allo-HCT) in children, comprising both patients with high-risk disease in first complete remission (CR1) or in relapsed phase (rALL) (3). Two basic types of high-dose conditioning therapy before HCT for ALL are used both in children and adults, based on total body irradiation (TBI) or on chemotherapy (CHT), mainly with the use of busulfan or treosulfan (4, 5). Differences in the efficacy and short-and long-term safety between TBI and CHT-based transplantations is a matter of debate (4, 6, 7). A benefit in the outcome after the use of TBI has been observed in adults ( 6), but not univocally in children, as presented in a meta-analysis in 2011 (8). A recent large international prospective trial showed improved survival and lower relapse risk in patients following TBI+etoposide conditioning in comparison to CHT-based allo-HCT in children aged over 4 years (9). However, there are almost no real-world data on pediatric ALL-HCT (10).Herein, we present our experience with allo-HCT in pediatric ALL with respect to the type of conditioning. The objective of this analysis was comparison of transplant outcomes: overall survival (OS), event-free survival (EFS), relapse incidence (RI), and graft-versus-host-disease (GVHD)-3315 This article is freely accessible online.