Ninety infants and young children with acute gastroenteritis were investigated for lactose malabsorption. Each of them was given an oral lactose load of 2g per kg of body weight after which breath hydrogen excretion was measured, and each was observed for clinical symptoms of lactose intolerance. Only 2 patients, given 2g per kg of lactose, had clinical lactose intolerance. Forty-nine of the 90 patients studied were found to have the rotavirus antigen in their stools. Forty-five of them were found to have an abnormal lactose breath hydrogen test (LBHT). Twenty-three patients with abnormal LBHT were restarted on a diluted lactose-containing formula for oral feeding. They required longer hospitalization (mean 6.7 days, range 3-14 days) and were free of diarrhea in 14 days (mean 7.5 days). Twenty-two patients found to have an abnormal LBHT were given a nonlactose-containing formula (Isomil, Nursoy, Alsoy, ProSobee, or Bebelac FL) when restarted on oral feeding. All patients require less than 5 days of hospitalization and free of diarrhea in 5 days (mean 3.4 days). The difference was statistically significant (p less than 0.05).