Background: The biophysical properties and etiology the of the non-relaxing upper esophageal sphincter (UES) are unknown. Our aims, in patients with manometrically confirmed failed UES relaxation, were to examine: 1) the etiology and biomechanical properties of the UES, 2) the relationship between UES opening and failed relaxation and determinants of sphincter opening. Methods: We examined the relationships among swallowed bolus volume, hypopharyngeal intrabolus pressure, sagittal UES diameter, the pharyngeal swallow response, and geniohyoid shortening in 18 patients with failed UES relaxation and compared them with 23 healthy aged controls and 15 patients with Zenker's diverticulum. Results: From 374 combined videoradiographic and manometric studies we identified 18 (4.8%) with failed UES relaxation which was attributed to: medullary disease (56%); Parkinson's or extrapyramidal disease (33%); and idiopathic (11%). The extent of UES opening ranged from absent to normal and correlated with the degree of preservation of the pharyngeal swallow response (P=0.012) and geniohyoid shortening (P=0.046). Intrabolus pressure was significantly greater when compared with aged controls (P<0.001) or Zenker's (P<0.001). A significant (P< 0.0001) bolus volume-dependent increase in intrabolus pressure, evident in the two control groups, was not observed in failed UES relaxation. Conclusions: The non-relaxing UES, displays a constant increased resistance to bolus flow throughout the full, and potentially normal, range of expansion during opening. Adequacy of UES opening is influenced by the degree of preservation of the pharyngeal swallow response and hyolaryngeal traction. In contrast, the UES with a fixed stenosis displays a fixed loss of compliance which is only operative once the limit of sphincter expansion has been reached.