The aim of the present study was to compare the gastric juice ammonia test to the CLO test for the diagnosis of H. pylori infection in culture-proven cases by receiver operating characteristic (ROC) curve analysis. We studied 75 subjects (44 with chronic gastritis, 10 with gastric ulcer, 6 with duodenal ulcer, 8 with gastric cancer, and 7 normal) by endoscopy with biopsy for tissue diagnosis, culture of H. pylori. CLO test, and by gastric juice ammonia determinations. The culture-positive group had significantly higher intragastric ammonia levels (13.7 +/- 5.8 mg/dl) than the negative group (4.9 +/- 2.4 mg/dl, P < 0.01). In ROC curve analysis, the gastric juice ammonia test showed higher true positive and lower false positive ratios than the CLO test (P < 0.05). In conclusion, the measurement of intragastric juice ammonia levels was considered to be simpler, quicker, and overall a more valuable method for diagnosing H. pylori infection.