Desmoid tumours (DT) are myofibroblastic proliferations found in 10–25% of familial adenomatous polyposis (FAP) patients, either intra-abdominally (IA), in the abdominal wall (AW) or elsewhere (extra-abdominal (EA)). Most DT occur following prophylactic colectomy but occasionally patients present with pre-operative DT. Mutations 3′ to codon 1444, predispose to DT and attenuated phenotype, leading to a potential strategy of delaying surgery in patients at high risk of DT. Here we assess if the existence of a pre-operative AW-DT or EA-DT predisposes to IA-DT following laparotomy. Data were collected on FAP patients undergoing primary surgery at St Mark’s Hospital. Patients were stratified into those having no DT, EA-DT or AW-DT pre-operatively. Relative-risks were calculated and proportions compared using Fisher’s exact-test. 587 FAP patients were identified; nine discovered with IA-DT intra-operatively were excluded. 5(0.9%) and 6(1%) of the remainder had a pre-operative EA-DT and AW-DT, respectively; one (0.2%) had both. Six of these 12 developed a post-operative IA-DT. 566(98%) had no pre-operative DT, 50(9%) of these developed IA-DT post-operatively; median time to tumour was 2 (IQR, 1–3.5) years. A pre-operative AW-DT predisposed to IA-DT post-operatively (RR = 7.6, 95% CI 4.0–14.1, P = 0.0009) whilst EA-DT did not (RR = 2.3, 0.4–13.3, P = 0.38). A 3′ mutation was significantly associated with post-operative IA-DT in the ‘no pre-operative DT’ group (P = 0.002). The presence of a DT external to the abdominal cavity pre-operatively, poses a clinical challenge with regards to surgical decision-making. Patients with an AW-DT pre-operatively may warrant a conservative surgical approach, to minimize post-operative IA-DT risk.