Small bowel adenocarcinoma is a rare condition with poor prognosis. Like colorectal cancer, small bowel carcinoma may be a part of genetic syndromes with carcinogenetic pathways different from sporadic forms. We report a case of 41-year-old man with small bowel carcinoma revealing hereditary non polyposis colorectal cancer (HNPCC) syndrome. This report supports the systematic study of the MSI status in every patient with a small bowel carcinoma below 60-year-old of age in order to screen for HNPCC syndrome even in the absence of a family history of related cancers.