• We studied the usefulness of the barium enema to rule out Hirschsprung's disease (congenital megacolon) in the evaluation of infants with constipation. Results from barium enemas in 58 constipated infants and children who underwent rectal suction biopsies to rule out aganglionosis were evaluated in retrospect for the presence of a transition zone, delayed evacuation of barium, and colonic anatomic abnormalities. As expected, the presence of a transition zone was most accurate in predicting Hirschsprung's disease. Eighty percent of infants with aganglionosis had a roentgenographic transition zone, while 20% did not. Twenty-nine percent of infants with a suspected roentgenographic transition zone did not have aganglionosis. Delayed evacuation of barium was a poor predictor of aganglionosis. The presence of delayed evacuation of barium did not improve the accuracy of the transition zone to predict Hirschsprung's disease. Barium enemas did not reveal any additional colonic anatomic defects other than a transition zone to account for constipation in our study group. These results demonstrated that the barium enema is not a specific enough screening procedure to rule out Hirschsprung's disease in the unobstructed infant. Rectal suction biopsy is diagnostic and should be performed as the procedure of choice in those infants suspected of having Hirschsprung's disease.