Objective: Fructooligosacharides and galactooligosacharides soften fecal bolus and increase frequency of depositions when added to infant formula. This study aimed to determine the effects of galactooligosaccharide in pediatric patients with chronic constipation. Methods: From 2010 to 2012, 20 constipated patients (4-16 years of age) attended to at a primary healthcare unit were enrolled in a double-blinded, placebo-controlled crossover trial. Eleven children ingested galactooligosaccharide (1.7 g) for 30 days, followed by a 15-day washout period, and a 30-day period of placebo (maltodextrin). Nine patients ingested maltodextrin for 30 days, followed by 15-day washout period, and galactooligosaccharide (1.7 g) for 30 days. Constipation symptoms were considered as primary outcomes: bowel movements/week, straining during defecation, and stool consistency. Outcome symptoms were ranked according to a numerical scale elaborated for this study. Data were recorded at baseline, and on days 15 and 30 of each 30-day crossover period. Repeated-measures analysis of variance (ANOVA) was used to analyze symptoms along time. Results: At baseline, there was no significant difference in symptoms severity between groups (p = 0.45). Galactooligosaccharide ingestion was related to increase of the bowel movement frequency, p < 0.0001; relief of defecation straining, p < 0.0001; and decrease in stool consistency, p = 0.0014, compared to placebo ingestion. Patients reported no side effects from galactooligosaccharide. Conclusion: Galactooligosaccharide was effective at improving clinical symptoms in this group of constipated children. (C) 2015 Sociedade Brasileira de Pediatria. Published by Elsevier Editora Ltda. All rights reserved.916 567573