We examined a case of congenital idiopathic megaesophagus (CIM) in a 5-wk-old female Gordon Setter puppy by means of contrast radiography, autopsy, histopathology, and immunohistochemistry. Clinical and radiologic findings included weight stagnation and marked generalized esophageal dilation with ventral displacement of the heart and lungs. These findings were confirmed at autopsy, and segments of the thoracic esophagus were sampled for histopathology. On histopathology, diffuse esophageal muscular atrophy, mucosal erosions, mononuclear inflammation, and a marked reduction in the number of myenteric plexus structures and number of ganglion cells were present (aganglionosis). The latter was determined immunohistochemically using an anti-peripherin antibody as the primary reagent, which provides a strong tool for the histologic confirmation of CIM. The histologic findings share some similarities to lesions associated with megaesophagus in Friesian foals, as well as esophageal achalasia and Hirschsprung disease in humans.