Objetive: To determine the diagnostic validity of an ammonia breath test for Helicobacter pylori infection in patients who undergo an upper gastrointestinal endoscopy at Hospital Cayetano Heredia. MATERIAL AND METHODS: From April to December 2014, 155 patients were evaluated with the ammonia breath test and compared with a histological evaluation of the gastric biopsies as the gold standard. Data were evaluated using Microsoft Excel and STATA 14 to build a ROC curve. RESULTS: The patients were predominantly female (71%), with a median age of 53 years (18-84) and a Helicobacter pylori prevalence of 51.6%. The ammonia breath test, when compared to the gastric biopsy has a 70% sensitivity, 36% specificity, 53.8% positive predictive value, 36% negative predictive value, 1.15 positive likelihood ratio and 0.75 negative likelihood ratio. According to the ROC curve, there is not an optimal cut off value and the area under the curve was 0.5517. CONCLUSIONS: The ammonia breath test evaluated on this study does not have diagnostic accuracy and is not recommended as a diagnostic tool for Helicobacter pylori infection.