The IgG antibody response specific to Helicobacter pylori was evaluated through ELISA in a group of 92 gastric patients colonized by this bacteria. 74 had gastritis and 19 gastroduodenal ulcer. Three control groups were studied in a similar way: normal adult volunteers (n = 17), adults with E coli or S typhi bacteremia (n = 30) and normal infants (n = 30). IgG antibody response to H pylori was demonstrated in 98% of colonized patients and 0% of infants. Asymptomatic individuals and those with bacteremia had high rates of antibody response (76 and 90% respectively), although this rate and also the titers of antibody response were significantly lower than that of colonized patients (p < 0.05). ELISA reactive sera from colonized patients and asymptomatic individuals evidenced a similar antibody pattern when tested by blotting. This profile was absent in non reactive sera, including those with high antibody titers to C jejuni. The presence of specific IgG antibodies to H pylori in the majority of colonized gastric patients and asymptomatic adults suggests that this infection is very common in our population.