In order to examine the effect of Helicobacter pylori colonization on the gastric mucus microclimate, antral juxtamucosal pH was measured in 47 patients attending as out patients for upper gastrointestinal endoscopy. The mean pH in 28 patients negative for H. pylori was 6.40 +/- 0.24 compared to 6.88 +/- 0.16 in 19 patients who were positive (P < 0.0001). In six of seven patients who agreed to a second study, H. pylori was eradicated and the mean pH fell from 6.81 +/- 0.17 to 6.08 +/- 0.16 (P < 0.001). The pH remained high in the one patient who remained positive (6.8 and 7.0). This study provides the first in vivo evidence that H. pylori can increase the antral juxtamucosal pH and suggests that ammonia production by the organism is capable of altering gastric mucus microclimate to impair the normal negative feedback controlling gastrin release. This observation may explain the coexistence of relative hypergastrinemia and H. pylori colonization in duodenal ulcer patients.