Introduction: Adenomatous polyps in the gastrointestinal system rarely occur in childhood and are accompanied by syndromes such as Familial adenomatous polyposis, attenuated familial adenomatous polyposis, and MUTYH-associated polyposis, Gardner and Turcot syndrome, and also mismatch repair (MMR) gene defects. In this article, we want to present a rare patient who had adenomatous polyposis and in situ carcinoma and was detected biallelic MMR gene defect. Case: A 16-year-old female patient admitted with painless rectal bleeding, chronic abdominal pain, and anorexia for 1 year. Her physical examination was notable for multiple cafe au lait spots. The colonoscopic and histopathologic examination revealed multiple adenomatous polyps that one of them contains low-high grade dysplasia and in situ carsinoma. Genetic analysis revealed a homozygous mutation in the PMS2 gene [c.1164delT (p.H388Qfs*10) (p.His388GInfsTer10)] and she was diagnosed with constitutional MMR gene defect syndrome. Polypectomy was performed 4 times in 2 years period. Then, the patient's last colonoscopic examination revealed a large broad polyp in the rectum and multiple polyps in the other colon segments, and she underwent colectomy because of high risk of colorectal cancer. Conclusions: Adenomatous polyps are very important in childhood because of rarity. In particular, the presence of cafe au lait spots and a history of malignancy detected in relatives at an early age must be considered for CMMRD.