Fifteen patients hospitalized with acute, watery diarrhea and with enterotoxigenic Escherichia coli (ETEC) detected from stool samples were studied to evaluate the extent to which natural ETEC diarrhea induces local and systemic antibody responses to E. coli heat-labile toxin (LT), homologous lipopolysaccharide (LPS), and colonization factors (CFA/I and CFA/II). Specific IgA and IgG antibodies to LT, CFA I and II, and each patient's homologous LPS were determined by ELISA in serum, saliva, breastmilk, and intestinal lavage fluid. The majority of patients had greater than a twofold rise in local levels of IgA antibodies in the intestine: 80% of LT+ patients responded to LT, 63% of CFA+ patients responded to CFA, and 78% of all toxin-positive patients responded to the LPS of their infecting strain. Local antibody responses in the intestine were associated with responses in breastmilk and saliva, but relationships were not clear-cut, and the usefulness of these secretions as proxy measures of local intestinal antibody production remains unclear. Antibody responses in serum also occurred in most patients and were significantly more frequent in cases than in controls. This study demonstrates that natural ETEC disease results in local IgA responses to LT, CFA, and LPS in the gut and also in immune responses in breastmilk, saliva, and serum.