Cancer survivors exposed to anthracycline chemotherapy are at risk for developing cardiomyopathy, which may have delayed clinical manifestation. In a retrospective cross-sectional study, we evaluated the utility of cardiopulmonary exercise testing (CPET) for detecting early cardiac disease in 35 pediatric cancer survivors by examining the associations between peak exercise capacity (measured via percent predicted peak VO2) and resting left ventricular (LV) function on echocardiography and cardiac magnetic resonance imaging (cMRI). We additionally assessed the relationships between LV size on resting echocardiography or cMRI and percent predicted peak VO2since LV growth arrest can occur in anthracycline-exposed patients prior to changes in LV systolic function. We found reduced exercise capacity in this cohort, with low percent predicted peak VO2(62%, IQR: 53–75%). While most patients in our pediatric cohort had normal LV systolic function, we observed associations between percent predicted peak VO2and echocardiographic and cMRI measures of LV size. These findings indicate that CPET may be more sensitive in manifesting early anthracycline-induced cardiomyopathy than echocardiography in pediatric cancer survivors. Our study also highlights the importance of assessing LV size in addition to function in pediatric cancer survivors exposed to anthracyclines.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00246-023-03192-z.