Abstract OBJECTIVES: In analogy with proton pump inhibitors, H2- antagonists may also be responsible for false-negative results on urea breath test for the detection of Helicobacter pylori. In this study we assessed the frequency and duration of false-negative urea breath tests in patients given different doses of ranitidine. METHODS: A total of 120 consecutive dyspeptic patients infected with H. pylori on the basis of concomitant positive results of CLO-test, histology and urea breath test were recruited for this prospective, open, parallel-group study performed in an urban university gastroenterological clinic. They were randomized to receive an acute treatment with either ranitidine 300 mg once a day in the evening, ranitidine 300 mg once a day in the morning, ranitidine 150 mg b.i.d., or ranitidine 300 mg b.i.d. for 14 days. The urea breath test was performed on day 14 while patients were still taking ranitidine, and on day 21, 1 wk after completion of therapy. The test was repeated on day 28 in those patients who were still negative on day 21. Duplicate breath samples were collected after ingestion of 75 mg 13C-urea plus citric acid. A δ value >5‰ was considered positive. RESULTS: Of 118 patients infected with H. pylori, 15 (13%) had a negative urea breath test on day 14. The false-negative results were equally distributed among the four groups of ranitidine dosage. Nine of these patients reverted to positive at 7 days and the remaining six at 14 days after completion of therapy. CONCLUSIONS: Our study shows that ranitidine negatively affects the results of urea breath testing, independent of the given dosage. Patients undergoing this examination for H. pylori diagnosis should discontinue use of H2-antagonists 2 wk before testing.