The recognition of Helicobacter pylori (H. pylori) as a major cause of gastroduodenal diseases has led to the use of antibiotics to treat these diseases. However, antibiotics used alone are not very effective, and adjuvant therapy is required. The most potent adjuvant therapy consists of increasing the stomach pH with proton pump inhibitors (PPIs). In addition to this action on stomach pH, PPIs, and especially lansoprazole, have been found to have antimicrobial activity against H. pylori. At high concentrations, they are even bactericidal. Furthermore, they can inhibit H. pylori urease activity. These properties, as well as their antisecretory activity, provide the grounds for their use in eradication of H. pylori.