The recognition that peptic ulcer is an infectious disease caused by the bacterium Helicobacter pylori has revolutionized the approach to diagnosis and therapy of this condition. Treatment of the symptoms of peptic ulcer with drugs that block acid secretion is already being replaced by antibiotic eradication of the causative agent. Studies of the molecular events that lead to H. pylori pathogenesis have shown that clinical isolates can be divided into two groups, only one of which produces a cytotoxin and is associated with severe disease. The cloning of the genes coding for molecules specific for disease-associated strains of H. pylori, and the development of animal models that mimic the human pathology, will provide the basis for better strategies to treat and prevent peptic-ulcer disease.