The competency of the cardia depends on the interaction of the distal esophageal sphincter (DES) pressure and the length of the DES exposed to the positive-pressure environment of the abdomen. These two components were measured in 20 normal control volunteers and 126 patients with objectively proved gastroesophageal reflux. The results, when plotted on a grid with the horizontal bar representing the length of the abdominal esophagus and the vertical bar representing the DES pressure, indicated that factors in addition to the mechanical components of the cardia were important in the antireflux mechanism. The 24-hour esophageal pH records from the patients and the antireflux mechanism. The 24-hour esophageal pH records from the patients and normal subjects were analyzed as to the number of reflux episodes that occurred per hour while the patients were in the supine position and the ability to clear the refluxed acid by the propulsive "P"pump" of the body of the esophagus. It was concluded that the antireflux mechanism of the esophagus consists of a valvular cardia and a propulsive "pump" action of the body of the esophagus. The failure of either may lead to abnormal acid exposure but can be compensated by one or the other in normal subjects. Failure of both invariably leads to abnormal acid exposure. The cardia can fail either mechanically (i.e., having inadequate valvular components) or functionally (i.e., having normal valvular component but abnormal number of reflux episodes per hour). The latter suggests gastric pathology. Precise diagnosis of the reason for abnormal acid exposure is needed to develop a rational basis for therapy.