Antibody responses to Helicobacter pylori were measured by a solid-phase whole-cell enzyme-linked immunosorbent assay in 150 children and adolescents; in 47 consecutive children undergoing upper gastrointestinal endoscopy, including 17 with H. pylori infection before and after antimicrobial treatment; and in 46 family members of the infected children. Abnormal levels of either IgG or IgA were found in 6% of the 150 children. In the latter group the prevalence of H. pylori seropositivity increased with age. Parents and siblings of the infected children had 94% and 71% seropositivity, respectively, suggesting intrafamilial spread. Abnormal levels of IgG or IgA against H. pylori identified infected children with 95% sensitivity and 84% specificity. Eradication of the infection was accompanied by a significant decrease in IgG and IgA titers, with normalization in 10 cured patients in 12 months or less. We conclude that the method described for evaluation of H. pylori-specific IgG and IgA antibodies gives helpful information on the epidemiology of the infection and represents a useful adjunct to diagnosis and management of chronic gastritis in children.