The analysis of esophageal manometry and 24 hour esophageal pH monitoring in 266 consecutive patients indicates that the competency of the cardia depends upon the amplitude of the distal esophageal high pressure zone and the length of the abdominal esophagus. These two determinants of competency were examined using human esophagi in a unique in vitro model which allowed control of these parameters, as well as intraabdominal, intragastric, and intrathoracic pressures. The following principles of the function of the abdominal esophagus were graphically illustrated: (1) Competency of a segment of intraabdominal esophagus without intrinsic tone occurs only when intraabdominal pressure is equal to or greater than intragastric pressure. (2) Competency of a segment of intraabdominal esophagus without intrinsic tone is directly related to its length. (3) The length of intraabdominal esophagus necessary to maintain competency is indirectly related to variations in intraabdominal pressure. (4) Competency of a segment of intraabdominal esophagus is augmented by the presence of intrinsic tone, and the shorter the length, the greater the intrinsic tone needed. (5) Competency of a segment of intraabdominal esophagus is augmented by negative intrathoracic pressure. These findings beautifully illustrate the mechanical valvelike function of the abdominal esophagus and the objectives to be accomplished in the surgical treatment of gastroesophageal reflux.