Although the pathogenic role of gastroesophageal reflux in Barrett's esophagus is widely accepted, the pattern of gastric and esophageal pH profile of patients with Barrett's esophagus is not well documented. Moreover, the observation that a columnar-lined esophagus can also develop after gastrectomy implies that chronic irritation of the lower esophagus by duodenal juice can be as harmful as acid reflux. To test this hypothesis, we simultaneously monitored gastric and esophageal pH in 19 patients with endoscopically and histologically proven Barrett's esophagus, in 35 with slight-to-moderate esophagitis and in 10 healthy subjects. The gastroesophageal reflux pattern in both Barrett's esophagus and esophagitis was characterized by mainly acid refluxes. Esophageal acid exposure (% time pH < 4) was 39.4 in patients with Barrett's esophagus, 14.6 in patients with esophagitis (P < 0.05), and 3.1 in healthy subjects (P < 0.05). Seven of 19 patients with Barrett's esophagus and 7 of 35 with esophagitis had evidence of alkaline reflux too; but pure alkaline refluxes accounted for only 1.9% of total time in Barrett's esophagus and 0.3% in esophagitis patients. In conclusion, these results confirm the high prevalence and severity of acid reflux in patients with Barrett's esophagus and show that the reflux of pure alkaline material into the esophagus is a rare event in both Barrett's esophagus and esophagitis patients.