BACKGROUND Probiotics are the most frequently prescribed treatment for children hospitalized with diarrhea in Vietnam. We were in uncertain of the benefits of probiotics for the treatment of acute watery diarrhea in Vietnamese children. METHODS We conducted a double blind, placebo-controlled, randomized trial of children hospitalized with acute watery diarrhea in Vietnam. Children meeting the inclusion criteria (acute watery diarrhea) were randomized to receive either two daily oral doses of 2 × 10 CFUs of a local probiotic containing Lactobacillus acidophilus or placebo for 5 days as an adjunct to standard-of-care. The primary endpoint was time from the first dose of study medication to the start of the first 24-hour period without diarrhea. Secondary outcomes included the total duration of diarrhea and hospitalization, daily stool frequency, treatment failure, daily fecal concentrations of rotavirus and norovirus, and Lactobacillus colonization. RESULTS 150 children were randomized into each study group. The median time from the first dose of study medication to the start of the first 24-hour diarrhea free period was 43 hours (inter-quartile range (IQR) 15-66 hours) in the placebo group and 35 hours (IQR 20-68 hours) in the probiotic group (acceleration factor 1.09 (95% confidence interval 1.78-1.51); p=1.62). There was also no evidence that probiotic treatment was efficacious in any of the pre-defined subgroups nor significantly associated with any secondary endpoint. CONCLUSIONS This was a large double blind, placebo-controlled trial in which the probiotic underwent longitudinal quality control. We found that under these conditions that Lactobacillus acidophilus was not beneficial in treating children with acute watery diarrhea.