Our objective was to determine the efficacy of probiotic use in reducing the duration of increased stool output in children with acute diarrheal illness. Eligible studies were limited to trials of probiotic therapy in otherwise healthy children <5 years old with acute-onset diarrhea. The main outcome variable was difference in diarrhea duration between treatment and control groups. Our meta-analysis of 18 eligible studies suggests that coadministration of probiotics with standard rehydration therapy reduces the duration of acute diarrhea by ~1 day [random-effects pooled estimate = -0.8 days (-1.1, -0.6), P < 0.001]. Differences in treatment effect between studies was assessed by calculating the Q statistic (Q = 204.1, P < 0.001). In subsequent analyses limited to studies of hospitalized children, to double-blinded trials, and to studies evaluating lactobacilli, the pooled estimates were similar (-0.6 to -1.2 days, P < 0.001). In conclusion, bacterial probiotic therapy shortens the duration of acute diarrheal illness in children by approximately one day.