ABSTRACT: BACKGROUND: Irritable bowel syndrome (IBS) is a common gastrointestinal disorder in children. Recently, probiotics have been suggested as a treatment option for gastrointestinal disorders. The most effective species and the most appropriate doses are still unknown. OBJECTIVE: The aim of this study was to assess the effects of Lactobacillus GG (LGG) for treating IBS in pediatric patients. METHODS: In a controlled, double blind, randomized trial, patients with IBS diagnosed by Rome III criteria from August 2012 to September 2012 at Dr. Sheikh Hospital, Mashhad University of Medical Sciences, Iran, were assigned to one of two groups, i.e., intervention and control groups. For four weeks, the intervention group received a probiotic in capsule form that contained LGG at a concentration of 1×10(10) cfu/ml bacteria. For the same period, the control group received a placebo capsule that had the same shape and color but only contained inulin, which also was present in the LGG capsules. The primary outcome was any change in the severity of the patients' pain, and we used a five-point Likert scale to evaluate the severity of their pain. Secondary outcomes were ghanges of the functional scale, stool patterns, and associated problems. RESULTS: Fifty-two patients participated in the study, and 26 patients were assigned randomly to each of the two groups. The severity of the patients' pain decreased significantly in the intervention group after one, two, three, and four weeks of treatment, as indicated by P-values of 0.01, 0.00, 0.00, and 0.00, respectively. Also, there was significant improvement in the functional scale after two weeks of treatment (P-value ≤ 0.00). CONCLUSION: Lactobacillus GG at a concentration of 1×10(10) cfu/ml for a period of four weeks can lessen the severity of the patients' pain and improve the functional scale in patients with irritable bowel syndrome. Probiotics can have therapeutic effects for IBS patients.