Background and purpose - Controversies exist regarding thromboprophylaxis in orthopedic surgery. Using data in the nationwide Norwegian Hip Fracture Register (NHFR) with postoperative death and reoperation in the first 6 months after surgery as endpoints in the analyses, we determined whether the thromboprophylaxis in patients who undergo hemiarthroplasty for femoral neck fracture should start preoperatively or postoperatively. Patients and methods - After each operation for hip fracture in Norway, the surgeon reports information on the patient, the fracture, and the operation to the NHFR. Cox regression analyses were performed with adjustments for age, ASA score, gender, type of implant, length of surgery, and year of surgery. Results - During the period 2005-2014, 25,019 hemiarthroplasties as treatment for femoral neck fractures were reported to the registry. Antithrombotic medication was given to 99% of the patients. Low-molecular-weight heparin predominated with dalteparin in 57% of the operations and enoxaparin in 41%. Only operations with these 2 drugs and with known information on preoperative or postoperative start of the prophylaxis were included in the analyses (n = 20,241). Compared to preoperative start of thromboprophylaxis, postoperative start of thromboprophylaxis gave a higher risk of death (risk ratio (RR) = 1.13, 95% CI: 1.06-1.21; p < 0.001) and a higher risk of reoperation for any reason (RR =1.19, 95% CI: 1.01-1.40; p = 0.04), whereas we found no effect on reported intraoperative bleeding complication or on the risk of postoperative reoperation due to hematoma. The results did not depend on whether the initial dose of prophylaxis was the full dosage or half of the standard dosage. Interpretation - Postoperative start of thromboprophylaxis increased the mortality and risk of reoperation compared to preoperative start in femoral neck fracture patients operated with hemiprosthesis. The risks of bleeding and of reoperation due to hematoma were similar in patients who received low-molecular-weight heparin preoperatively and in those who received it postoperatively.