During a 2-year prospective study of gastroenteritis in children less than 2 years of age, the role of enteric adenovirus as a cause of infantile diarrhea was examined in three clinical settings in a case-control fashion. Using a monoclonal antibody-based enzyme-linked immunosorbent assay with specificity for adenovirus serotypes 40 and 41, enteric adenovirus was identified in 10 of 246 episodes of diarrhea in outpatients (4.1%), 13 of 211 children admitted to the hospital with diarrhea (6.2%), and 5 of 81 children in whom nosocomial diarrhea developed (6.2%), making this agent the third most commonly identified etiologic agent of diarrheal disease. Asymptomatic infections were uncommon (5 of 372 control subjects, or 1.3%) and were seen most frequently in the nosocomial setting. Cases occurred in every calendar month except March and April of each year. A syndrome of watery diarrhea of longer duration compared with other patients with diarrhea (mean 5.4 vs 3.8 days, P = .01), associated with vomiting and dehydration, was present in most cases. Compared with patients with rotavirus, patients were as likely to experience fever and dehydration and more likely to vomit. Household contact with gastroenteritis, often with a child 2 to 5 years of age, was a predisposing factor. It was concluded that enteric adenovirus is an important cause of infantile diarrhea in Baltimore children. Although far less common than rotavirus, this agent was associated with diarrheal illnesses that were at least as severe as those seen with rotavirus.