To evaluate the mother-child transmission of anti-Helicobacter pylori antibodies, we investigated 562 pregnant women by means of a commercially available second-generation enzyme-linked immunosorbent assay for the detection of anti-H. pylori IgG (Malakit Helicobacter pylori). One hundred twenty-eight of the 562 women had a positive serology for H. pylori. 13C-Urea breath tests were performed in 85 seropositive and in 65 randomly selected seronegative subjects. These breath tests were positive in 82 of the 85 (96.5%) seropositive and in none of the seronegative subjects, reflecting the actual presence of H. pylori in the gastric mucosa of the seropositive women. Cord blood levels for the Malakit Helicobacter pylori were positive in all infants born to seropositive mothers and negative in those born to seronegative mothers. In all infants the previously positive titers had turned negative by the age of 3 months. At the age of 12 to 15 months 13C-urea breath tests were performed in 67 infants born to seropositive mothers. These breath tests were positive in only one infant. By the time the 13C-urea breath tests were performed, the serology had turned negative in all infants with the exception of the one with the positive breath test. We conclude that the IgG antibodies against H. pylori cross the placental barrier and that, despite the present H. pylori infection in the mothers, infants born to these H. pylori-positive women do not appear to have an increased risk of developing a H. pylori-associated gastritis during the first year of life.