OBJECTIVE: To systematically evaluate the effectiveness of probiotics in preventing antibiotic-associated diarrhea (AAD) in children. STUDY DESIGN: The following electronic databases up to December 2005, in any language, were searched for studies relevant to AAD and probiotics: MEDLINE, EMBASE, and The Cochrane Library. Only randomized controlled trials (RCT) were considered for study inclusion. RESULTS: Six placebo-controlled, RCTs (766 children) were included. Treatment with probiotics compared with placebo reduced the risk of AAD from 28.5% to 11.9% (relative risk, RR, 0.44, 95% CI 0.25 to 0.77, random effect model). Preplanned subgroup analysis showed that reduction of the risk of AAD was associated with the use of Lactobacillus GG (2 RCTs, 307 participants, RR 0.3, 95% CI 0.15 to 0.6), S. boulardii (1 RCT, 246 participants, RR 0.2, 95% CI 0.07-0.6), or B. lactis & Str. thermophilus (1 RCT, 157 participants, RR 0.5, 95% CI 0.3 to 0.95). CONCLUSIONS: Probiotics reduce the risk of AAD in children. For every 7 patients that would develop diarrhea while being treated with antibiotics, one fewer will develop AAD if also receiving probiotics.