The prevalence of IgG antibodies to Helicobacter pylori was determined with an enzyme-linked immunosorbent assay in 80 families who lived together with 40 duodenal ulcer patients in whom Helicobacter pylori had been cultured from a gastric biopsy (34 spouses, 31 children, 10 parents, 4 sisters and 1 brother) and in 112 controls from the same habitat and with similar age. The antibodies were positive in 38.4% of the relatives and in 36.6% of the controls, the difference was not significant. Among spouses of patients, 38.4% of those aged 25-39 years and 66.6% of those aged 40-67 years were positive, whereas controls showed a 29.2% and a 58.3% of positives respectively. The differences between both groups were not significant. Among children, 17.2% were positive and in parents 50%, whereas among controls with a similar age 26.3% and 62.5% respectively were positive. The differences between relatives and controls were not significant. We conclude that in our environment among consanguineous families living together and between spouses, person-to-person spread of Helicobacter pylori does not usually occur or it happens uncommonly.