Background: Well-controlled clinical studies in the developed world have shown that probiotics can shorten the duration of acute non-bacterial diarrhea. We aimed to evaluate the efficacy of a probiotic consisting of a mixture of Lactobacillus acidophilus and Bifidobacterium bifidum in the treatment of young children with acute diarrhea in Mashad, Iran Methods: Sixty-two hospital inpatients aged 6 to 36 months with acute non-bloody, non-bacterial diarrhea of a less than 2 days' duration and moderate dehydration were enrolled. Thirty-two were treated with probiotic powder three times daily for 5 days plus the routine oral rehydration solution (study group) and the other 30 were given a placebo plus oral rehydration solution (control group). Results: Mean age at the time of admission was 14.5±7 months for the study group and 13.7±6 months for controls. The mean duration of diarrhea was 3.4 days in the study group and 4.5 days in controls (P = 0.027). Duration of hospital admission was 2.1±0.7 days in the probiotic group compared with 2.7±0.6 days in the control group (P = 0.033). Average weight gain was 425 ± 9 and 370 ± 85 g for the study and control groups, respectively. Average reduction in the frequency of diarrhea per day was 4.4 ± 1.5 times for the study group and 3.6 ± 1.3 times for the control group. Conclusion: Lactobacillus acidophilus and Bifidobacterium bifidum shortened the duration of diarrhea and hospital stay, and normalized stool frequency. The use of probiotics might be recommended for treating acute diarrhea in young children.