During early summer 1975 and spring 1976, outbreaks of acute gastroenteritis were reported from primary schools and other institutions in several districts of Japan. Outbreaks occurred in an explosive manner resembling mass food poisoning from a school lunch. The majority of patients were in the age group 6-14 years. Clinical features were generally mild, consisting of vomiting and/or diarrhea, often with low-grade fever. Reovirus-like agents in the feces were found in 27 (44%) of 62 patients. The virus found in feces of schoolchildren with acute gastroenteritis (SCGV) was related morphologically as well as serologically, not only to the agent found in infantile gastroenteritis (IGV), but also to neonatal calf diarrhea virus (NCDV). A slight difference in antigenicity between SCGV and IGV as suggested by cross complement fixation (CF) remains to be elucidated. About one-half of paired sera from 54 patients showed a significant rise in CF antibody against SCGV and/or NCDV. The pattern of neutralizing (NT) antibody against NCDV in patients' sera was similar to that of CF antibody. Most children studied had a titer of 1:4 or greater of CF and/or NT antibodies to SCGV and NCDV in acute sera. The relationship between acute gastroenteritis associated with reovirus-like agent in infants and that in schoolchildren is discussed.