Although H. pylori is well established as an etiological agent of type B gastritis and a predisposing factor for peptic ulcer, knowledge about its transmission is unclear. In this study we examined the prevalence of H. pylori infection in the family members of index infants infected with this organism as indicated by positive [13C]-urea breath test (UBT). We performed UBT among family members of 15 predominantly breastfed infants, eight with and seven without H. pylori infection. Infection rates were 82% and 91% in family contacts of the infected and noninfected infants respectively, the average infection rate being 85%, which is rated to be high. There was no difference in infection rates among the parents of the infected and noninfected infants. Fifty percent and 70% families belonging to infected and noninfected infants, respectively, were found to have all members infected with H. pylori. No evidence of sex predilection of infection was found. We conclude that in communities with high prevalence of H. pylori infection, there is almost an equal infection rate among the family contacts of infected and noninfected infants, suggesting that environmental factors may be more important than intrafamilial transmission.