The results of a prospective study of 20 cases of newly diagnosed Hirschsprung's disease (nine of whom developed enterocolitis) and 10 normal controls showed no variations in the bacterial flora (including Clostridium difficile) in the stools of the groups studied. Viral studies showed that rotavirus was present in the stools of seven of the nine cases of enterocolitis during the episode. We suggest that Hirschsprung's enterocolitis may have a complex infective aetiology and that rotavirus plays a part.