This study examined the effects of eradication of Helicobacter pylori (H pylori) infection on gastric mucosal morphology and acid secretion. Sixteen H pylori positive patients with enlarged gastric body folds were divided into two groups: (a) patients with moderate enlargement (fold width: 6 to 10 mm, n = 8) and (b) patients with severe enlargement (> 10 mm, n = 8). After successful treatment, gastric body fold width was reduced in both groups (p < 0.01) with an associated decrease in inflammatory infiltrates in the body mucosa (p < 0.01 and p < 0.05). Basal acid output and tetragastrin stimulated maximal acid output (mean (SEM)) in all 16 patients significantly increased from 1.1 (0.5) to 2.9 (0.9) mmol/h (p < 0.05) and from 5.4 (1.3) to 18.7 (2.3) mmol/h (p < 0.01), respectively, with a significant decrease in fasting serum gastrin concentrations, from 127.1 (16.1) to 59.6 (3.8) pg/ml (p < 0.01). The increase in acid secretion after eradication of H pylori was more noticeable in the severe group, who had shown lower acid secretion and higher serum gastrin concentrations (p < 0.05) before eradication, than the increase seen in the moderate group. The decreases in ammonia nitrogen content seen after eradication were significant in basal (from 0.91 (0.17) to 0.37 (0.08) mmol/h, p < 0.05) and stimulated gastric secretions (from 1.57 (0.19) to 0.37 (0.13) mmol/h, p < 0.01), although these changes were too small to explain the increases in basal acid output and maximal acid output. These results suggest that inflammation of the gastric body mucosa caused by H pylori infection is associated with enlarged gastric body folds and inhibition of acid secretion in H pylori positive patients with enlarged gastric body folds.