Purpose: Allogeneic Stem Cell Transplant (allo-HSCT) patients are at risk of malnutrition and weight loss from impaired oral intake resulting from gastrointestinal toxicities, dysgeusia, and psychological effects.Methods: A retrospective review of 264 adult patients transplanted at Princess Margaret Cancer Centre who achieved relapse-free survival up to 3 months after allo-HSCT was performed.Results: Overall incidence of patients who experienced WL (WL) ≥10% from HSCT to 3 months posttransplant was 45.9% and from HSCT to 6 months was 56.6%. Patients with ≥10% WL from allo-HSCT at 3-months and 6 months had similar 2-year overall survival (OS) compared to those with <10% WL, 55.7% vs. 62.8% (HR=1.38, p=0.11) and 71.1% vs. 77.2% (HR=1.37, P=0.27), respectively. Patients with ≥10% WL 3-and 6-months from allo-HSCT also had similar 2-year relapse-free survival (RFS) compared to those with <10% WL, 48.1% vs. 55.8% (HR=1.26, p=0.22), and 62.7% vs 69.8% (HR=1.29, p=0.31), respectively.The 2-year transplant-related mortality (TRM) was higher for those with ≥10% WL from allo-HSCT to 3months, 35.4% vs. 16.9% (HR=2.39, p=0.0007) and 6 months, 22% vs. 8% (HR=3.1, p=0.0034). Although statistical signi cance was not observed for OS or RFS, patients who experienced ≥10% WL 3-and 6months post allo-HSCT experienced higher 2-year TRM. These results highlight the importance of early intervention and close monitoring of weight post allo-HSCT. Conclusion: Approaches to WL post allo-HSCT should be multifaceted and include members of the interdisciplinary team in order to decrease TRM.