© 2015 by European Society for Pediatric Gastroenterology, Hepatology, and Nutrition and North American Society for Pediatric Gastroenterology, Hepatology, and Nutrition.Objectives: In the present study, we studied a cohort of patients with very early onset inflammatory bowel disease (IBD) to determine the frequency of mutations in the interleukin 10 (IL10) receptor genes as a cause of earlyonset IBD. Methods: Sanger sequencing was performed to determine the presence of IL10 and/or IL10 receptor mutations in 17 patients with a diagnosis of very early onset IBD (disease onset <2 years of age in 15 patients, between 3 and 4 years in the other 2). Mutation screening was performed including all of the coding regions of the IL10, IL10RA, and IL10RB genes. We then compared the follow-up findings of the patients with IL10 receptor mutations in terms of demographic, clinical, laboratory, and treatment response properties with those of patients diagnosed as having very early onset IBD with no mutation. Results: We identified 3 patients bearing mutations in the IL10 or IL10 receptor genes, including 1 mutation in IL10RB that has been described recently (c.G477A, p.Trp159∗) and 2 novel mutations affecting the IL10RA gene (c.T192G, p.Tyr64∗ and c.T133G, p.Trp45Gly). Collectively, these mutations thus provided genetic etiology for 17.6% of the cohort under investigation. The presence of a family history of IBD and the clinical course of Crohn disease differed between patients with mutations in the IL-10 pathway and those without such mutations. Although perianal fistulas were found in all of the patients with IL10 receptor mutations, they were found in only 14.3% of those without such mutations. The lower values of weight-forage and height-for-age z scores, necessity for more intensive therapy, achievement of longer periods until remission, and frequent relapses in the patients bearing mutations in the IL10 receptor genes all underlined the severity of the disease and its relatively poor response to treatment. Conclusions: In spite of the small number of patients with mutations affecting the IL-10 signaling pathway in our study, in all of the patients with IL10 receptor mutations, the disease onset occurs at an early age, the prognosis is poor, and the response to treatment is insufficient.