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Simple Summary: Abdominal desmoids are rare fibroblastic tumors. Though these tumors do not display metastatic potential, their locally aggressive nature can cause severe outcomes. Most cases appear sporadically, but 5–15% are associated with familial adenomatous polyposis (FAP) syndrome. Current consensus recommendations do not offer a standard sequence of therapy due to the lack of data for some treatment options. Here, we present an ongoing clinical experience with abdominal desmoids. The majority of our patients suffered severe outcomes such as need for surgery or major tumor complications. A small, but unique group of 16 non-FAP mesenteric desmoid was found to harbor genetic alterations in cancer associated genes other than APC, including CHEK2, BLM, ERCC5, MSH6, and PALB2. Introduction: Abdominal desmoid tumors are locally aggressive tumors that develop in familial adenomatous polyposis (FAP) patients, within the mesentery or abdominal wall. The understanding and implications of the treatment regimens are evolving. Aim: To assess the course, treatment, and outcomes of FAP and non-FAP abdominal desmoids and their related genetic alterations. Methods: Retrospective cohort study. Demographics, tumor characteristics, oncological and surgical history, complications, genetic-testing, and mortality data were retrieved from two tertiary referral centers. Results: Sixty-two patients were identified (46 FAP and 16 non-FAP). Thirty-eight patients (61.3%) underwent surgical procedures (12 urgent and 26 elective). Out of 33 tumor resections, 39.4% recurred. Hormonal therapy, COX-inhibitors, chemotherapy, imatinib, and sorafenib were used in 35 (56.4%), 30 (48.4%), 18 (29.1%), 7 (11.3%), and 8 (12.9%) of patients, respectively, with a 2 year progression-free survival of 67.8%, 57.7%, 38.4%, and 28.5%, respectively. Forty-one patients (66.1%) suffered complications: bowel obstruction (30.6%), hyperalimentation (14.5%), ureteral obstruction (12.9%), perforation (11.3%), abscess formation (3.2%), and spinal cord compression (3.2%). Non-FAP patients carried pathogenic mutations in CHEK2, BLM, ERCC5, MSH6, and PALB2. Conclusions: Abdominal desmoids are mostly FAP-related and are associated with severe outcomes. We also report a group of non-FAP abdominal desmoids, which includes patients with additional cancer-related gene alterations. This interesting group should be further explored. [ABSTRACT FROM AUTHOR]