A band of increased density paralleling the left lower border of the cardio-pericardial silhouette is seen on anteroposterior chest films in some children with pericardial effusions. To analyze the incidence and accuracy of this sign, films of children with pericardial effusions and controls were compared. Among 17 children with proved pericardial effusion, bands of density within the cardio-pericardial silhouette were seen in six films, while positive epicardial fat pad signs were seen on four. When three radiologists without experience in use of the sign evaluated radiographs of the six patients with the sign and six controls without pericardial effusion, the sign was detected in most instances in the children with effusions (specificity and sensitivity, 83%). In vitro studies of unpreserved hearts in baths of solutions of varying osmolarity failed to show detectable differences in density between myocardium and any of the solutions. We conclude that the new sign may be produced by epicardial fat adjacent to pericardial fluid.