AIM To examine childhood correlates ofHelicobacter pylori infection in adults. DESIGN Follow up study of men and women whose birth weight, weight at age 1 year, and feeding in infancy were recorded by health visitors. Data on childhood housing conditions were obtained by recall. SUBJECTS 631 men and 389 women born in Hertfordshire during 1920–30 and still living in the east or northwest districts of the county. MAIN OUTCOME MEASURES Serum H pyloriIgG antibodies measured by enzyme linked immunosorbent assay. RESULTS Independent of their current social class, subjects were more likely to be H pylori seropositive if they had large numbers of siblings (p < 0.0001), and if they had lived in a crowded house (p = 0.001), or shared a bedroom or bed in childhood (p = 0.02). Low weight at 1 year was associated with increased seropositivity rates in men (p = 0.0002), but not women (p = 0.8). Men and women who were breast fed in infancy were less likely to be seropositive than those who were bottle fed (p = 0.08). CONCLUSIONS The findings support the current view that H pylori infection is often acquired in childhood by close person to person contact, and persists into adult life. H pylori infection may be a cause of failure to thrive in infancy, especially in boys. Alternatively, small infants may be more susceptible to infection. Breast feeding may prevent early infection.