To determine the usefulness of heart size on chest radiograph (CXR) in predicting cardiac enlargement (CE) in children, we prospectively evaluated 95 consecutive outpatients, who had both a CXR and echocardiography performed. Their median age was 5.0 years (2 days to 19.9 years). All patients underwent CXR assessment by a pediatric radiologist, with classification of cardiac silhouette as normal, borderline, or enlarged. Echocardiographic assessment of CE was performed by a pediatric echocardiographer. Sensitivity, specificity, and predictive values of the pediatric radiologist's interpretation of heart size on CXR were estimated. The presence of CE by echocardiography was used as the gold standard. Seventy-nine patients (83.2%) had no CE on CXR, and 16 patients (16.8%) had CE. Sensitivity of the CXR to identify CE was 58.8%, 95% confidence interval (CI) [32.9, 81.6], with a positive predictive value of 62.5% [35.4, 84.8]. Specificity was 92.3% [84.0, 97.1], with a negative predictive value of 91.1% [82.6, 96.4]. These data suggest that the assessment of CE on CXR to predict CE by echocardiography has a relatively high specificity and negative predictive value, but a low sensitivity and positive predictive value. The limitations of CXR as a diagnostic test should be understood by clinicians using the test when screening children for cardiac disease.