Abstract： Objective Many previous meta-analysis have shown that the probiotics can lower the incidence of antibiotic-associated diarrhea (AAD) in children.However,the function and efficacy of probiotics drugs showed obvious strains specificity and dose dependence.Currently,most of the reported meta-analysis regarding probiotics AAD prevention have adopted Saccharomyces,Lactobacilleae,Streptococcus and Bifidobacterium.This study aimed to evaluate the efficacy and safety of the live Clostridium butyricum and Bifidobacterium Powder to prevent AAD in hospitalized children with pneumonia.Method This study was a multicenter,randomized,controlled clinical trial; 380 hospitalized children with pneumonia aged from 3 months to 3 years were enrolled from April to Dec.2011.Totally 372 children completed the study,179 children in control group received antibiotics as routine treatment for pneumonia;193 children in treatment group received 5 × 109 colony-forming units of Clostridium butyricum and Bifidobacterium combined Powder daily for 7 days during the antibiotics treatment.The stool frequency and consistency (assessed according to the Bristol Excrement Assessment Scale) were observed for 7 consecutive days; the incidence of diarrhea and adverse drug reactions were recorded.Result Both treatment and control groups were similar in age distribution,sex,type of antibiotics,route of administration,and time of antibiotics used.During the 7 days period,the rate of AAD was 7.8％ (13/193) in treatment group and 16.8％ (30/179) in control group,there was significant differeuce; compared with the control group,the treatment with Clostridium butyricum and Bifidobacterium combined Powder can lead to 53.6％ reduction in AAD risk (RR =0.419,95 ％ CI 0.217-0.808;P =O.008).The severity of diarrhea was comparable in both study groups,as evidenced by similar stool frequency,dehydration,fever and vomiting.There was significant difference (P =0.008) in the AAD occurrence time distribution between the treatment group and control group.No drug related adverse reactions (ADRs) were found during the observation period.Conclusion The live Clostridium butyricum and Bifidobacterium combined Powder may effectively reduce the risk of AAD in hospitalized children with pneumonia,and no ADRs were found.