Introduction: Heparin is a safe, effective approach to preventing venous thromboembolism. The objectives of this study were to identify the rates and types of thromboprophylaxis used in the ICU. Hypothesis: We hypothesized that approximately 80% of patients would receive some anticoagulation, that mechanical prophylaxis would be infrequent, and that low molecular weight heparin (LMWH) would be used less than unfractionated heparin (UFH). Methods: We conducted a retrospective audit of thromboprophylaxis in patients admitted to 14 ICUs in November 2011, documenting actual anticoagulant and mechanical thromboprophylaxis prescribed for the duration of ICU stay, and reasons why no anticoagulant was prescribed. Results: We enrolled 1083 patients from 12 Canadian and 2 US centers, age [mean (sd)] 62.0 (17.1) years with an APACHE II score of 18.3 (7.7). Mortality was 9.5% in ICU and 16.9% in hospital. Patients received prophylactic anticoagulation with UFH (54.9%) or LMWH (23.3%), whereas 20.9% of patients were therapeutically anticoagulated (with UFH, coumadin, LMWH or danaparoid). Overall, 82.9% received some form of anticoagulation. For 1503/6245 (24.1%) patient-days in ICU when no anticoagulation was administered, reasons were high risk of bleeding (43.4%), bleeding (16.6%), late admission or discharge (10.8%), no reason evident (10.2%), a procedure (9.2%), perception that it was unnecessary (6.6%), life support limitation (6.3%) and suspected or proven heparin-induced thrombocytopenia (1.6%). Mechanical prophylaxis was used less often (5.2% patient-days for anti-embolic stockings and 22.6% patient-days for pneumatic compression devices); however, mechanical prophylaxis was most common among patients never receiving anticoagulation (3.5% and 60.6% patient-days, respectively). Conclusions: In this one month multicenter ICU audit, we documented widespread use of anticoagulation (prophylactic or therapeutic), greater use of UFH than LMWH, dual use of anticoagulant and mechanical prophylaxis in a minority of patients, and frequent mechanical prophylaxis in patients who are bleeding or at risk of bleeding. Funding: the Hamilton Academy of Health Sciences