An enzyme immunoassay for the detection of serum IgG antibodies to H. pylori whole bacterial sonicate was used in a study of dyspeptic adult patients (n=165), apparently healthy blood donors (n=154), and children (n = 66) from Costa Rica, where a very high incidence of gastric carcinoma is observed. The mean antibody levels and frequency distributions were similar in the dyspeptic patients and the blood donors. Meanwhile lower antibody levels to H. pylori were more frequently observed in children than in adults, suggesting an age-dependent increase in seroprevalence, as described in studies from other populations.