Twenty-four pH monitoring the distal esophagus quantitates gastroesophageal reflux in a near physiologic setting by measuring the frequency and duration of acid exposure to the esophageal mucosa. Fifteen asymptomatic volunteers were studies with 24-hour pH and esophageal manometry. The normal cardia was more competent supine than in the upright position. Physiologic reflux was unaffected by age, rarely occurred during slumber, and was the rule after alimentation. One hundred symptomatic pateitns with an abnormal 24-hour pH record (2 S.D. above the mean of controls) could be divided into three patterns of pathological reflux: those who refluxed only in the upright position (9), only in the supine position (37), and in both positions (54). Upright differed from supine refluxers by excessive aerophagia causing reflux episodes by repetitive belching. Compared to controls, they had excessive post-prandial reflux, lower DES pressure, and less DES exposed to the positive pressure of the abdomen. Supine differed from upright refluxers by having a higher incidence of esophagitis and an inability to clear the esophagus of acid after a supine reflux episode. Compared to controls, they had only a lower DES pressure. Combined refluxers had a higher incidence of esophagitis than supine refluxers. Stricture (15%) was seen only in this group. They were similar to supine refluxers in their inability to clear a supine reflux episode. Compared to controls, they had a lower DES pressure and less DES exposed to the positive pressure of the abdomen. Forty of the 100 patients had an antireflux procedure (4 upright, 8 supine, 28 combined). The most severe postoperative flatus and abdominal distention was seen in the upright refluxers. It is concluded that minimal reflux is physiological. Patients with pathological reflux all have lower DES pressure. Patients with upright reflux have less of their DES exposed to the positive pressure environment of the abdomen. Patients with supine reflux have an inability to clear the esophagus of reflux acid and are prone to develop esophagitis. Patients with both upright and supine reflux have the most severe disease and are at risk in developing strictures. In patients with only upright reflux, aerophagia and delayed gastric emptying may be an important etiological factor.