Objectives: International guidelines recommend thrombosis prophylaxis after total hip arthroplasty (THA) and total knee arthroplasty (TKA) for up to 35 days. However, previous studies often have hospital stays (length of stay; LOS) of 8-12 days and not considering early mobilisation, which may reduce incidence of venous thromboembolic events (VTE). We investigated the incidence of any symptomatic thromboembolic events (TEEs) with only in-hospital prophylaxis if LOS ≤5 days after fast-track THA and TKA. Design: A prospective descriptive multicentre cohort study in fast-track THA and TKA from February 2010 to December 2011, with complete 90-day follow-up through the Danish National Patient Registry and patient files. Setting: 6 Danish high-volume centres with a similar standardised fast-track setup, including spinal anaesthesia, opioid-sparing analgesia, early mobilisation, functional discharge criteria and discharge to own home. Participants: 4924 consecutive unselected unilateral primary THA and TKAs in patients ?18 years with no preoperative use of continuous 'potent' anticoagulative therapy (vitamin K antagonists). Exposure: Prophylaxis with low-molecular-weight heparin or factor Xa-inhibitors only during hospitalization when LOS ≤5 days. Outcomes: Incidence of symptomatic TEE-related, VTErelated and VTE-related mortality ≤90 days postoperatively. Results: LOS ≤5 days and thromboprophylaxis only during hospitalisation occurred in 4659 procedures (94.6% of total). Median LOS and prophylaxis duration was 2 days (IQR 2-3) with 0.84% (95% CI 0.62% to 1.15%) TEE and 0.41% (0.26% to 0.64%) VTE during 90-day follow-up. VTE consisted of five pulmonary embolisms (0.11% (0.05% to 0.25%)) and 14 deep venous thrombosis (0.30% (0.18% to 0.50%)). There were four (0.09% (0.04% to 0.23%)) surgery-related deaths, of which 1 (0.02% (0.00% to 0.12%)) was due to pulmonary embolism, and 6 (0.13% (0.06% to 0.28%)) deaths of unknown causes after discharge. Conclusions: The low incidence of TEE and VTE suggests that in-hospital prophylaxis only, is safe in fasttrack THA and TKA patients with LOS of ≤5 days. Guidelines on thromboprophylaxis may need reconsideration in fast-track elective surgery. Trial Registration: ClinicalTrials.gov: NCT01557725.