Background: Cardiovascular disease (CVD) is a leading cause of morbidity and mortality after allogeneic stem cell transplantation (SCT), yet effective risk stratification and prevention remain hindered by reliance on resting echocardiographic measures of left-ventricular function, which often fail to detect subclinical dysfunction. While evaluating cardiorespiratory fitness (VO2peak) has proven effective in unmasking cardiac dysfunction across the SCT care continuum, its time, cost, and expertise demands may limit widespread clinical adoption, highlighting the need for surrogate predictors. Objective: This study aimed to examine the relationship between VO2peak and echocardiographic measures of cardiac structure, which are established as strong independent predictors of VO2peak in other populations, in adults across the SCT care continuum. Methods: A total of 175 adults (96 SCT candidates, 62 early SCT survivors, 14 long-term SCT survivors) underwent 1) cardiopulmonary exercise testing to quantify VO2peak, and 2) resting echocardiography to quantify body surface area indexed left atrial systolic volume (LAVi), left ventricular mass (LVMi), and left-ventricular end-diastolic volume (LVEDVi). The relationships between VO2peak and candidate measures of resting cardiac structure were assessed using univariate, and multivariate linear regression adjusted for age and sex. Results: VO2peak showed a very weak association with LAVi (R2=0.04; standardised β: -0.19, p=0.13) and LVEDVi (R2=0.04; standardised β: 0.19, p=0.010), but both remained significant independent predictors of VO2peak after adjusting for age and sex (standardised β: -0.39, p<0.001; standardised β: 0.34, p<0.001). No significant association was found between VO2peak and LVMi (R2=0.01; standardised β: -0.07, p=0.38). Conclusion: Compared with previous reports in broader populations, LVEDVi, LAVi and LVMi demonstrated significantly lower predictive ability for VO2peak across the SCT care continuum, making them unreliable surrogate predictors of cardiorespiratory fitness and CVD risk in this population.