Serum samples from 1397 Ecuadorian children 0–5 years of age were tested by ELISA for antibodies to enterotoxigenic Escherichia coli (ETEC). A gradual prevalence and titer increase was seen for IgG antibodies to heat-labile enterotoxin in children 6–18 months old; 90% prevalences were reached in the second year of life. At this age <10% of West German children showed IgG antibodies to heat-labile enterotoxin. In Ecuador age-related ETEC-specific antibody titer increases correlated with age-related prevalence of diarrheal disease. On the other hand, pooled lipopolysaccharide from different ETEC-associated 0 serogroups could not be used as a seroepidemiologic marker of ETEC infections.