Lynch syndrome is a genetic disorder related to cancer predisposition, including colorectal cancer, endometrial cancer, and ovarian cancer. Germline mutations in mismatch repair genes, including MLH1, MSH2, MSH6, and PMS2, are responsible for this condition. Cancer tissue specimens resected from small bowel adenocarcinoma in a Japanese patient showed decreased expression of MLH1 and PMS2 by immunohistochemistry testing. Finally, a novel MLH1 mutation, c.1833dup, was identified in this patient.