Colorectal carcinoma in the population aged less than 20 years of age is rare but associated with poor prognosis, which is attributable to advanced disease at presentation and higher incidence of the unfavourable mucinous histology. Colorectal carcinoma commonly presents with non-specific gastrointestinal symptoms in conjunction with iron deficiency anaemia. Many of these symptoms can mimic eating disorders, which are common in adolescent women. We present the case of a 20-year-old woman with previously undiagnosed familial adenomatous polyposis and colorectal carcinoma who experienced a significant delay in diagnosis, given a coexisting eating disorder mimicking her symptoms. After confirmation of the diagnosis by colonoscopy and genetic testing, the patient underwent a successful proctocolectomy and experienced full recovery. This case is a reminder that underlying organic pathology should always be excluded prior to a diagnosis of an eating disorder.