Maintenance of gastric juxtamucosal pH at a stable near neutral value may be the cumulative effect of the various components of the mucosal defense system. In order to assess the effect of helicobacter pylori (HP) infection on mucosal defense, we measured the gastric antral juxtamucosal pH in 40 dyspeptic patients by using a flexible glass pH microelectrode which can be passed down the instrument channel of standard gastroscopes. HP status was determined using serology, culture, histology and urease test. We also investigated the relationship between juxtamucosal pH and the severity of antral HP infection. The mean antral juxtamucosal pH in 26 (65%) HP positive patients was 6.49 +/- 0.20 compared to 6.19 +/- 0.21 in 14 (35%) HP negative patients (p < 0.00001). Other factors like age, sex, duodenogastric reflux or presence of chronic duodenal ulcer did not significantly affect juxtamucosal pH (p > 0.05). Subset analysis of data on HP positive patients (n = 26) revealed no significant correlations between antral chronic gastritis anti-HP IgG titre and antral juxtamucosal pH (p > 0.05). This study shows that HP increases gastric antral juxtamucosal pH. This finding supports the suggested role of HP in producing hypergastrinaemia and gastric acid hypersecretion.