Household members in California USA were tested for H. pylori infection. Among 1,752 person considered uninfected at baseline, 30 new infections (7 definite, 7 probable, and 16 possible) occurred. Exposure to an infected household member with gastroenteritis was associated with a 4.8-fold increased risk for definite or probable new infection. Of probable/definite new infections, 75% were attributable to exposure to an infected person with gastroenteritis. The authors suggest that person-to-person transmission of H. pylori is most commonly implicated with fecal/oral, oral/oral, or gastric/oral pathways; infection is associated with conditions of crowding and poor hygiene and intrafamilial clustering. Emerging Infectious Diseases 2006, 12, 1701-1208.