We present a case of a 25-year-old female with diagnosed familial adenomatous polyposis and elevated carcinoembryonic antigen with negative family history. The suspicion of Gardner's syndrome was raised because extirpation of an osteoma of the left temporo-occipital region was made 10 years ago. Restorative procto-colectomy and ileal pouch anal anastomosis was made but histology delineated adenocarcinoma of the rectum (Dukes C stage). We conclude that cranial osteomas often precede gastrointestinal manifestations of familial adenomatous polyposis or Gardner's syndrome and such patients should be evaluated with genetic testing followed by colonoscopy if results are positive to prevent the development of colorectal carcinoma. If the diagnosis is positive all family members should be evaluated for familial adenomatous polyposis.