The objective of this study was to determine the efficacy of Lactobacillus casei sps. rhamnosus (Lactobacillus GG) (LGG) in reducing the incidence of antibiotic-associated diarrhea when coadministered with an oral antibiotic in children with acute infectious disorders. Two hundred two children between 6 months and 10 years of age were enrolled; 188 completed all phases of the protocol. LGG, 1 x 10(10) - 2 x 10(10) colony forming units per day, or comparable placebo was administered in a double-blind randomized trial to children receiving oral antibiotic therapy in an outpatient setting. The primary caregiver was questioned every 3 days regarding the incidence of gastrointestinal symptoms, predominantly stool frequency and consistency, through telephone contact by blinded investigators. Twenty-five placebo-treated but only 7 LGG-treated patients had diarrhea as defined by liquid stools numbering 2 or greater per day. Lactobacillus GG overall significantly reduced stool frequency and increased stool consistency during antibiotic therapy by the tenth day compared with the placebo group. Lactobacillus GG reduces the incidence of antibiotic-associated diarrhea in children treated with oral antibiotics for common childhood infections.