Background: Survivors of childhood cancer exposed to cardiotoxic therapies are at significant cardiovascular risk. The utility of cardiac biomarkers for identifying the risk of future cardiomyopathy and mortality is unknown. Methods: N-terminal pro–B-type natriuretic peptide (NT-proBNP) and cardiac troponin T (cTnT) were assessed in 1213 adults 10 or more years from a childhood cancer diagnosis; 786 were exposed to anthracycline chemotherapy and/or chest-directed radiation therapy (RT). NT-proBNP values above age- and sex-specific 97.5th percentiles were considered abnormal. Generalized linear models estimated cross-sectional associations between abnormal NT-proBNP and anthracycline or chest RT doses as risk ratios with 95% confidence intervals (CIs). A Poisson distribution estimated rates and a Cox proportional hazards model estimated hazard ratios (HRs) for future cardiac events and death. Results: At a median age of 35.5 years (interquartile range, 29.8-42.5 years), NT-proBNP and cTnT were abnormal in 22.5% and 0.4%, respectively. Exposure to chest RT and exposure to anthracycline chemotherapy were each associated with a dose-dependent increased risk for abnormal NT-proBNP (P for trend <.0001). Among exposed survivors with no history of Common Terminology Criteria for Adverse Events–graded cardiomyopathy and with normal systolic function, survivors with abnormal NT-proBNP had higher rates per 1000 person-years of cardiac mortality (2.93 vs 0.96; P <.0001) and future cardiomyopathy (32.10 vs 15.98; P <.0001) and an increased risk of future cardiomyopathy (HR, 2.28; 95% CI, 1.28-4.08) according to a multivariable assessment. Conclusions: Abnormal NT-proBNP values were prevalent and, among survivors who were exposed to cardiotoxic therapy but did not have a history of cardiomyopathy or current systolic dysfunction, identified those at increased risk for future cardiomyopathy. Further longitudinal studies are needed to confirm this novel finding.