Enteric types of adenovirus have recently been identified as a causative agent of infantile gastroenteritis. We utilized enzyme immunoassay and tissue culture techniques to evaluate prospectively the role of ET Ad in diarrhea occurring in hospitalized infants. We found that ET Ad was associated with 14 of 27 cases of diarrhea occurring during a 12-week study period in the late autumn and early winter months; ET Ad was found in the stool of only one of 72 children without diarrhea (P less than 0.001). Although adenoviruses other than ET Ad were found in the stools of two of the 27 children with diarrhea, such viruses were also found in the stools of five of 72 children without diarrhea and thus could not be statistically correlated with acute gastroenteritis. Children infected with ET Ad had diarrhea for a mean of 8.0 days, compared to a mean duration of 4.2 days for the children with gastroenteritis not associated with ET Ad. Thirteen of the 14 children with ET Ad gastroenteritis had respiratory symptoms such as cough, rhinorrhea, or wheezing, six had roentgenographic evidence of pneumonia, and three children had bilateral conjunctivitis. This study documents that ET Ad can be an important cause of acute gastrointestinal disease in hospitalized infants and young children and that gastrointestinal infections with ET Ad can be associated with a high rate of respiratory disease.