We conducted a 19-month prospective study of children attending 20 day-care centers to determine the occurrence, causes, and transmission of gastroenteritis among children, staff, and family members. Nine centers had 15 outbreaks of diarrhea involving 195 persons. A enteropathogen was identified in all outbreaks. Shigella was detected in five outbreaks, rotavirus in two, giardia in one, and in the remaining seven multiple enteropathogens were identified. Rotavirus and Giardia occurred only in children less than 3 years of age; shigellosis occurred at all ages. In six DCC 68 single cases of diarrhea were not associated with an outbreak; an enteropathogen was identified in only three (4%) persons. Thirty-four family members (11%) developed diarrhea associated with the occurrence of gastroenteritis in children in six DCC evaluated for this problem. Secondary attack rates of diarrhea in families according to organisms identified in the DCC outbreaks were: Shigella 26%, rotavirus 15%, and G. lamblia 17%. DCC may play an important role in the epidemiology and transmission of gastroenteritis in the United States.