Acute gastroenteritis (AGE) is a leading cause of malnutrition and death worldwide and affects millions of children in the US each year. Most AGE is viral in origin, and apart from care focused on rehydration and nausea, there are no therapies to reduce the severity of illness. The overall objective of this randomized, placebo-controlled, double-blind, multi-center study is to determine the efficacy and side effect profile of the probiotic, Lactobacillus rhamnosus GG (LGG), in children aged 3 to 48 months presenting to the ED with AGE. Patients were randomized to receive either LGG or placebo twice a day for 5 days, for a total of 10 doses. Nine-hundred and seventy-one children were enrolled in 10 PECARN sites over 4 years. Enrollment completed in Summer 2017. Analysis is in progress. This project is funded by the NICHD.