Achalasia is characterized by absent or incomplete lower esophageal sphincter (LES) relaxation and aperistalsis in the smooth muscle esophageal body in response to swallowing. The esophageal and LES response to distention has not previously been studied. I aimed to characterize the responses to esophageal balloon distention in achalasia patients in comparison to controls. Sixteen consecutive achalasia patients and 11 healthy volunteers underwent standard esophageal manometry followed by graded midesophageal balloon distention during which LES (as measured by the Dent sleeve) and esophageal body pressures were monitored. Subject perception of distention was also recorded using a standardized scoring system. The LES relaxation response to esophageal balloon distention was markedly impaired in achalasia patients, irrespective of whether the patient had radiological evidence of a dilated or nondilated esophagus. However, phasic contractions proximal to the distending balloon were preserved. The esophageal body responses below the balloon were inconsistent in both groups, and not significantly different from one another. Pain-sensation scores were significantly lower in achalasia patients at the highest distending volumes, but this difference was attributable to the subgroup of patients with a dilated esophagus. Distention-induced LES relaxation is markedly impaired in achalasia patients in keeping with loss of intrinsic inhibitory innervation. Preservation of the proximal excitation suggests that extrinsic vagal reflexes are intact.