Intra-abdominal desmoid tumours represent a major cause of morbidity and mortality in patients with familial adenomatous polyposis (FAP), and such patients are also liable to develop musculoskeletal desmoids. We have reviewed the CT appearances of 44 desmoid lesions (28 intra-abdominal and 16 musculoskeletal) in 20 patients with FAP. We found a considerable heterogeneity in the CT appearance of musculoskeletal and intraabdominal desmoids, with respect to their density, definition and change in size or density on follow-up, not only between different patients but also in patients with multiple lesions, who rarely showed identical appearances of all lesions. In some cases, mesenteric tumours may initially present as ill-defined soft tissue infiltration of mesenteric fat, becoming larger and more mass-like with time. On medical treatment, shrinkage was seen infrequently in musculoskeletal desmoids, and not at all with mesenteric lesions. CT evidence of bowel involvement by intra-abdominal lesions was frequent, most commonly appearing as 'tethering' or encasement of bowel loops. The presence of a large mesenteric mass (> 10 cm diam.), multiple mesenteric masses, extensive small bowel involvement and/or bilateral hydronephrosis were associated with ultimate death.