The preexisting levels of rotavirus IgA and IgG were measured in 225 children aged 6 months to 7 years in November, ie, before the "rotavirus season" from January to April. During the following 6 months, all episodes of acute gastroenteritis (GE) were evaluated clinically according to a score system and feces was examined for rotavirus, pathogenic bacteria, and parasites. Furthermore, rotavirus GE (n = 45) as well as asymptomatic rotavirus infections (n = 29) were diagnosed serologically. The preexisting concentrations of rotavirus IgA and IgG measured by ELISA were similar in these two groups. However, preexisting rotavirus IgA in the group of children who developed rotavirus GE correlated with less severe symptoms. Thus vomiting was found in 24% and 63% of the children with detectable and undetectable rotavirus IgA, respectively (P less than 0.025). Moreover, according to the total symptom score of rotavirus GE, 52% of the children with detectable preexisting rotavirus IgA had mild symptoms compared with only 13% of those with undetectable concentrations (P less than 0.025). Rotavirus IgG did not have any protective effect. Age per se had a protective effect; older age (greater than 1.5 years) was related to mild symptoms. According to previous studies of local and intestinal antibody response to a rotavirus GE, it is suggested that rotavirus IgA in serum reflects the immunological status of the intestine concerning rotavirus. It is recommended that studies of rotavirus vaccines include rotavirus IgA response and its protective effect.