We studied 26 patients (11 males and 15 females) undergoing elective surgery under general anesthesia. The purpose of this study was to decide how long the length of nasopharyngeal airway should be by measuring distance A (permitting airway obstruction to be released), distance B (giving the most effective ventilation), and distance C (between nostril and arytenoid). The values of distance A in male group and female group were 12.73 +/- 0.85 cm and 11.70 +/- 0.75 cm, respectively. The values of distance B were 14.55 +/- 0.96 cm in male group and 13.93 +/- 1.12 cm in female group. The values of distance C were 18.84 +/- 0.90 cm in male group and 17.40 +/- 0.97 cm in female group. This showed that it is necessary to advance the nasopharyngeal airway about 2 cm from the distance A to give the most effective ventilation to the patients with airway obstruction. Therefore, most of standard nasopharyngeal airways commercially available are too short. In addition, the distance B has no correlation with height and body weight and it is difficult to predict the optimal length of the airway.