Twenty-four Helicobacter pylori (H. pylori)-positive patients were treated for 28 days with either 20 mg omeprazole o.m. (n = 12) or 40 mg omeprazole o.m. (n = 12). Clearance (absence of H. pylori at the end of or shortly after treatment) and eradication (absence of H. pylori 1 month after cessation of treatment) were assessed using the 14C-urea breath test. Observed clearance and eradication were: 20 mg omeprazole 3/12 and 0/12; 40 mg omeprazole 6/12 and 1/12 respectively. The effect on H. pylori is probably due to the change in gastric pH from acid to neutral, however it is insufficient to recommend the inclusion of omeprazole in regimens aimed at eradicating H. pylori.