Background: It is now accepted that the curing of Helicobacter pylori infection will result in healing of chronic active gastritis and will change the natural history of gastroduodenal ulcer disease. Both endoscopic observation and evaluation of H. pylori status of the stomach are necessary for diagnosis and treatment of such patients. We carried out a clinical evaluation of an endoscopic tube type urease sensor system for the detection of H. pylori on the gastric mucosa. The differential output of two pH-sensitive field effect transistors at the tip of the endoscopic tube reflects the pH change in a urea solution depending on the existence of urease. Methods: In vitro experiments and clinical evaluation of the system were performed. Fifty-one patients who were suspected to have a gastroduodenal disorder were examined for H. pylori infection with this system, using the combination of histologic and bacteriologic examinations and rapid urease test as the references. Results: Clinical sensitivity and specificity of this system were 26 of 28 (92.9%) and 22 of 23 (95.7%), respectively. A measurement at 1 site is completed in about 1 minute. Repetition of the procedure provides multi-site measurements. Conclusions: The present system makes possible quick on-site detection of H. pylori under endoscopic observation, with satisfactory sensitivity and specificity. (Gastrointest Endosc 1999;49:32-8.)