To determine whether the urease-based CLOtest or low-dose 14C urea breath test can predict severity of gastritis or the presence of peptic ulcers, we studied 84 patients presenting for upper endoscopy. Antral biopsies were obtained for histologic analysis and CLOtesting, and urea breath testing was performed. The time to a positive CLOtest and the breath test peak values were correlated with endoscopic findings, severity of gastritis, and bacterial burden. Twenty-four patients had positive urea breath test results (22 with positive CLOtests). Patients with positive breath test results were more likely to have duodenal ulcers, higher grades of gastritis, and increased bacterial burden (p < 0.01 for all comparisons). Correlation between the time to a positive CLOtest or peak breath test values and gastritis severity or bacterial burden was poor (p > 0.05 for all comparisons). In patients with positive tests, no difference was observed between the time to a positive CLOtest or peak breath test value in patients with or without peptic ulcers (p < 0.2 for all comparisons). The low-dose 14C urea breath test and the CLOtest are both accurate for Helicobacter pylori diagnosis. However, neither test predicts the severity of the gastritis, the degree of bacterial burden, or the presence of peptic ulcers.