OBJECTIVE An ideal assay (inexpensive, sensitive, specific, and readily available) for Helicobacter pylori is lacking. Urease activity is an important characteristic of the organism and is employed in the rapid urease and urea breath tests. In this study, we assessed whether a simpler test, namely, measurement of gastric juice urease activity, would provide comparable results. METHODS Gastric juice was analyzed for urea and ammonia in 57 patients evaluated with rapid urease test and histology. Urease activity was assessed by the fraction of urea hydrolyzed to ammonia. RESULTS Thirty-five subjects were H. pylori positive and 22 were H. pylori negative. Compared with noninfected subjects, H. pylori-positive patients had lower urea levels (0.52 +/- 0.10 vs. 2.77 +/- 0.48 mM, p < 0.01), higher ammonia concentrations (6.59 +/- 1.06 vs. 1.64 +/- 0.25 mM, p < 0.01), and higher gastric urease activity (0.83 +/- 0.03 vs. 0.24 +/- 0.14, p < 0.01). In H. pylori-negative patients, there was a correlation between blood and gastric urea (r = 0.61, p < 0.01). However, in H. pylori-positive patients, no such relationship existed (r = 0.30, p = 0.11). The ratio of gastric to blood urea was lower in infected patients (0.11 +/- 0.02 vs. 0.45+/- 0.04, p < 0.01). The sensitivity and specificity of gastric urease activity for diagnosis of H. pylori were 91% and 100%, respectively, and for the ratio of gastric to blood urea, 89% and 95%, respectively. CONCLUSIONS Gastric juice urease activity is a simple, sensitive, and specific means to detect H. pylori.