Helicobacter pylori (H pylori) is likely the most common cause of chronic active gastritis in humans. Also, H pylori has been found in up to 100% of patients with peptic ulcer disease. Recent studies have shown that long-term infection by H pylori is associated with an increased risk of developing gastric carcinoma. The mechanism(s), however, by which H pylori causes gastritis or leads to the development of peptic ulcers and gastric cancer is not well understood. The prevalence of H pylori gradually increases with age and is much higher in underdeveloped countries. In the United States, H pylori is present in 50% to 60% of people 60 years of age and older. The prevalence of H pylori in African Americans in the United States is approximately 38% higher than that in whites in all age groups. The route of transmission of this organism is unknown, but it is most likely from person to person. H pylori infection has been rather difficult to eradicate. At present, the most effective antimicrobial therapy includes bismuth salts and two antibiotics plus an H2-receptor antagonist.