Pattern of gastric and esophageal acidity were evaluated in 14 patients with endoscopically and histologically proven Barrett's esophagus, in 46 with slight-to-moderate esophagitis, and in 22 healthy subjects. In patients with Barrett's esophagus, LES pressure was considerably lower and percentage exposure to acid was considerably higher than in either patients with esophagitis or controls (p less than 0.001). Percentage of time with esophageal pH below 4 was 33.2% in patients with Barrett's esophagus, 14% in patients with slight-to-moderate esophagitis (p less than 0.001), and 3.4% in controls (p less than 0.001). In patients with Barrett's esophagus, the esophageal exposure to acid was similar in upright and supine positions, and the number of refluxes that lasted longer than 5 min was also greater in these patients than in uncomplicated esophagitis or controls (p less than 0.001). Accordingly, their acid-clearing capacity was markedly reduced (p less than 0.001 vs. control). Omeprazole 20 mg, given once daily, reduced both percentage of time with pH below 4 (p less than 0.001) and the number of episodes longer than 5 min (p less than 0.001), but had no effect on acid clearance. In patients with Barrett's esophagus, omeprazole lowered intragastric acidity by 77.8% (p less than 0.001). Median intragastric pH was 1.9 (1.7-2.1) pretreatment, and 4.5 (4.2-5) during omeprazole (p less than 0.001).