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Desmoids occur in 15% of familial adenomatous polyposis (FAP) patients, 70% being intraabdominal desmoids (IAD). Most mortality is attributable to IAD; we identified risk factorspredicting IAD development.A multivariate analysis of our institutional database was performed. Hazard ratios (HR) were calculated for gender, 3′ APC mutation, type of surgery (ileorectal anastomosis or restorativeproctocolectomy), age at surgery and family history (FH) of desmoids.Five hundred and fifty-five patients were analyzed, 49 (9%) developed IAD; 22 (4%) diagnosed intra-operatively and 27 (5%) developing over a median post-operative period of 34 (7- 120) months. 75% of IAD developed before age 40. A 3′ APC mutation (HR5.2, 95%CI 2.1-13.3, P = 0.001), positive FH (HR2.5, 95%CI 1.4-4.6, P = 0.003) and female gender (HR1.9, 95%CI 1.0-3.5, P = 0.04) werepredictive of IAD development. No difference in IAD risk was detected between the type of surgical intervention (P = 0.37) or age at surgery (P = 0.29). No differences in characteristics were detected amongst patients with intra-operative or post-operative IAD development.3′ APC mutation is the most significant risk factor for IAD development. The independent association between positive FH and IAD risk supports the existence of modifier genes. As surgery often triggers IAD formation, delayingprophylactic surgery may be appropriate in highrisk patients.