Clinical desmoid disease affects approximately 10 per cent of patients with familial adenomatous polyposis (FAP); the subclinical rate is unknown. Desmoids are probably neoplastic rather than regenerative in origin and may arise in association with germline or somatic mutations at or beyond codon 1444 of the APC gene. Intra-abdominal desmoids behave unpredictably but are an important cause of death in those with FAP. Signal intensity on magnetic resonance imaging reflects tumour cellularity, which in part determines progression, and this may help management. Surgical treatment of advanced desmoids is hazardous, but medical treatments have limited success. Chemotherapy with doxorubicin and dacarbazine is currently under evaluation.