Few studies have examined the vagal afferent innervation of the esophagus in naturally occurring esophageal motility disorders. The present study assessed the integrity of distension-sensitive vagal afferents innervating the esophagus in naturally occurring canine megaesophagus. In the dog, esophageal distension induces reflex inhibition of crural diaphragm electromyographic activity that is mediated by vagal afferents innervating esophageal mechanoreceptors. This reflex was measured during stepwise esophageal distension in six dogs with congenital idiopathic megaesophagus, two dogs with megaesophagus secondary to esophageal striated muscle disease, and eight matched controls. In contrast to control dogs, inhibition of crural electromyographic activity was not observed in megaesophagus dogs with esophageal distension within the control volume range. With esophageal distensions far in excess of the control volume range, inhibition of crural electromyographic activity was not observed in five of six dogs with congenital idiopathic megaesophagus, while crural inhibition was observed in the two dogs with secondary megaesophagus. These findings indicate that a defect is present in the vagal afferent innervation to the esophagus in a majority of dogs with congenital idiopathic megaesophagus.