The authors present a case of colonic monoblastic sarcoma, previously treated for other digestive abnormalities (malabsorbtion, Hirschprung's disease). Important similitudes with immunoproliferative small intestinal disease (IPSID) lymphoma were demonstrated for this patient (male, 2-year-old). His main admission complaints were failure to thrive, recurrent episodes of enterocolitis and malabsorbtion syndrome. Some particularities of this case are the young age and the extremely rapid development of the malignant disease in a patient with no previous signs of acute non-lymphoblastic leukemia. The initial diagnosis was of malabsorbtion syndrome, based on the clinical exam at presentation, and then the patient was thought to have a form of Hirschprung's disease, due to a functional intestinal disorder (slow transit). After the necropsy, pathologists diagnosed an immunoproliferative small intestinal disease, and four years later, they performed a more appropriate pathological exam, which explained better clinical symptoms associated to this complex case.