Background: Venous thromboembolism (VTE) remains a prominent cause of morbidity and mortality following cancer surgery. Though ASCO evidence-based guidelines recommend major cancer surgery thromboprophylaxis start before incision and continue at least 7-10 days postoperatively, the extent to which the guidelines are followed is unknown. We assessed variation in thromboprophylaxis practices for abdominal cancer surgery in a regional surgical collaborative. Methods: We studied abdominal resections for primary gastrointestinal, hepatopancreaticobiliary (HPB) and neuroendocrine malignancies in the Michigan Surgical Quality Collaborative from July 2012-Sep 2013 (N=1,444 patients in 52 hospitals). We obtained detailed perioperative and postoperative pharmacologic and mechanical thromboprophylaxis information for patients without documented exemptions (e.g. active bleeding, allergy). We then compared differences in procedure mix and operative complexity across hospitals based on their perioperative thromboprophylaxis rates. Results: Overall, 43.1% of eligible patients had perioperative pharmacologic thromboprophylaxis for abdominal cancer surgery, and 25.3% (318/1258) of the highest-risk patients had evidence of inadequate postoperative prophylaxis (under-prophylaxis either by dose or duration). Hospital perioperative thromboprophylaxis rates ranged from 0%-96.1%, and postoperative thromboprophylaxis rates ranged from 73.9%-100%. Compared to hospitals with the lowest perioperative thromboprophylaxis rates, hospitals with the highest perioperative prophylaxis rates performed more HPB procedures (24.1% vs. 5.3%) and fewer concomitant operative procedures (31.1% vs. 44.7%). Epidural use did not impact hospital pharmacologic thromboprophylaxis rates. Conclusions: Fewer than half of patients undergoing abdominal cancer surgery receive perioperative thromboprophylaxis, and there is wide variation in hospital thromboprophylaxis utilization despite strong evidence-based guidelines supporting its use. Identifying reasons for non-adherence to published guidelines and best practices will be important to improve outcomes in this vulnerable patient population.