BACKGROUND Venous thromboembolism remains a prominent cause of morbidity and mortality following cancer surgery. Although evidence-based guidelines recommend major cancer surgery thromboprophylaxis starts before incision and continues 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, hepatopancreatobiliary (HPB), and neuroendocrine malignancies in the Michigan Surgical Quality Collaborative from July 2012 to September 2013 (N = 2967 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), and compared differences in procedure mix and operative complexity across hospitals based on their perioperative thromboprophylaxis rates. Additionally, we surveyed hospitals to identify variations in perioperative practice and barriers to prophylaxis administration. RESULTS Overall, 40.4 % of eligible patients had perioperative pharmacologic thromboprophylaxis for abdominal cancer surgery, and 25.3 % 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 to 96.1 %, and postoperative thromboprophylaxis rates ranged from 73.9 to 100 %. Epidural use was not independently associated with 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.