Gardner's syndrome is a clinical subgroup of familial adenomatous polyposis, an autosomal dominant disease. It is characterized by gastrointestinal polyps and extra-intestinal manifestations including multiple osteomas, skin and soft tissue tumours. Aggressive desmoid tumours can be very difficult to manage in patients with Gardner's syndrome. We present a case of a 17-year-old female who presented with an aggressive desmoid tumor arising of the lumbar area as part of her Gardner's syndrome. She was treated with surgery, nonsteroidal anti-inflammatory drugs, tamoxifen and radiotherapy, and was followed up for 80 months. We conclude that desmoid tumors can precede gastrointestinal manifestations of familial adenomatous polyposis or Gardner's syndrome. Such patients should be evaluated with genetic testing followed by colonoscopy. Desmoid tumours should be managed in a multidisciplinary setting, as well.