A new radiographic finding associated with feline congenital peritoneopericardial diaphragmatic hernias (CPDH) was identified and is presented in this paper. This finding represents the dorsal border of the hernia and when present appears on the lateral thoracic radiograph. Descriptively, the finding was called the dorsal peritoneopericardial mesothelial remnant (DPMR). In order to determine the recognizability and possible impact of this radiographic finding, thoracic radiographs of 10 cats with CPDH were randomly mixed with 20 other feline thoracic radiographic studies, which included 4 normal cats and 16 cats with a variety of primary cardiac disorders. Eight radiologists were instructed to identify the presence or absence of the DPMR. They were told that the DPMR connected the dorsal aspect of the caudal pericardium to the diaphragm at the site of the CPDH, and that the DPMR would be present on the lateral radiograph ventral to or superimposed over the area of the caudal vena cava. The reviewers were then asked to describe any other radiographic features that were consistent with a diagnosis of CPDH. At least 6 of the 8 reviewers identified the DPMR as being present in each of the 10 cats. In 17 instances, reviewers used the presence of the DPMR as the only radiographic finding in reaching a diagnosis of CPDH. In 4 instances of CPDH, the DPMR was not recognized by the reviewers, but other radiographic features were used that led them to a correct diagnosis of CPDH. In 2 separate instances, a reviewer misidentified pleural effusion/thickening within the interlobar fissure between the middle and caudal lung lobes as being the DPMR. In 7 false-positive diagnoses of CPDH, reviewers did not identify the presence of the DPMR but diagnosed CPDH based on perceived abnormalities in cardiac shape. Each of these cats had feline cardiomyopathy and not CPDH. It is concluded that in the absence of more typical thoracic radiographic features of feline CPDH, the presence of a DPMR on the lateral radiograph should make one suspicious of CPDH as the cause of cardiac silhouette enlargement or contour abnormality.