A 25-year-old male patient was admitted to our clinic for abdominal pain, diarrhea, intermittent rectal bleeding and weight loss. The family history revealed two deaths due to colorectal cancer (maternal grandmother and patient's mother). The colonoscopy showed hundreds of polyps throughout the colon, and an ulcerative rectosigmoidian tumor. The diagnosis was Familial Adenomatous Polyposis (FAP). Colectomy with ileorectal anastomosis was performed. Histopathological diagnosis revealed moderately differentiated adenocarcinoma. Adjuvant chemotherapy was carried out. The patient had three brothers, without clinical symptoms. They had a colonoscopic examination for screening. Two of them were diagnosed with adenomatous polyposis - the first with classic FAP and the other one with the attenuated type (AFAP). The diagnosis of FAP can be made on the basis of either clinical or genetic criteria. When the family history, clinical features, and pathological findings are classic, the diagnosis is straightforward. Screening and prophylactic surgery are effective to prevent colorectal cancer in patients with FAP. Lifelong regular surveillance is necessary to detect and manage extracolonic lesions.