Abstract Purpose: Cancer survivors’ exposure to chemotherapeutic agents leads to multiple long-term side effects with a decrease in their cardiorespiratory fitness. The first aim was to determine whether cardiorespiratory fitness and physical activity levels were lower among survivors than healthy Canadians, while the second aim was to report associations between genetic variants and cardiorespiratory fitness in survivors. Methods: Cardiorespiratory fitness (VO2peak) and moderate to vigorous physical activity (MVPA) were compared between childhood ALL survivors (N=221) and healthy Canadians (N=825). We performed whole-exome sequencing in survivors. Germline variants (both common and rare) in a selected set of trainability genes were analyzed for an association with cardiorespiratory fitness. Results: Survivors’ VO2 peak was found to be 22% lower than healthy Canadians. The cardiorespiratory fitness level was different between survivors and healthy Canadians despite a clinically equivalent level of MVPA. Positive associations between the cardiorespiratory fitness level and trainability genes (TTN, LEPR, IGFBPI, and ENO3 genes) were reported, especially in female survivors with a low cardiorespiratory fitness level. Conclusion: The cardiorespiratory fitness was significantly lower in survivors, which can be associated with variants in genes related to subjects’ trainability. At this time, it appears that more physical activity would be beneficial to survivors to achieve the same benefits as the healthy population. However, the optimal amount of physical activity needed to reach these benefits is not yet clear. The survivors’ responder or nonresponder status several years after the end of the treatments is unknown. This study has important implications for the field of exercise in oncology. Citation Format: Maxime Caru, Kateryna Petrykey, Mariia Samoilenko, Simon Drouin, Valérie Lemay, Laurence Kern, Lucia Romo, Patrick Beaulieu, Pascal St-Onge, Laurence Bertout, Geneviéve Lefebvre, Gregor Andelfinger, Maja Krajinovic, Caroline Laverdière, Daniel Sinnett, Daniel Curnier. The need to improve exercise prescriptions to support care in pediatric oncology [abstract]. In: Proceedings of the AACR Special Conference on the Advances in Pediatric Cancer Research; 2019 Sep 17-20; Montreal, QC, Canada. Philadelphia (PA): AACR; Cancer Res 2020;80(14 Suppl):Abstract nr B62.