Artificial nasopharyngeal airway position and performance were assessed in 120 anaesthetised adult patients. Using a fibreoptic laryngoscope mean distances from nares to larynx were measured at 209 mm (SD 11) in males and 180 mm (SD 11) in females; those from nares to epiglottis were 159 mm (SD 12) in males and 140 mm (SD 11) in females. Nasopharyngeal airways were frequently misplaced, 60% lying beyond the tip of the epiglottis and 13% lodged in the vallecula. Forty-two percent of subjects had clinical evidence of respiratory obstruction. Nasopharyngeal airway compression in the nasopharynx and obstruction by the tongue and soft palate were common causes of respiratory obstruction. Regression analysis revealed that nares-epiglottis length correlated significantly with subject height (t = 3.9, p = 0.0002), but not with three external measurements made around the head and neck. Head flexion and extension resulted in comparatively little relative movement of the nasopharyngeal airway. Nasopharyngeal airway length and diameter should be standardised to optimise performance.