Background: In non-cardiac arterial procedures (NCAP), heparin is administered to prevent arterial thromboembolic complications (ATEC). Heparin has a nonpredictable effect in the individual patient, also known as variation in heparin sensitivity. Various dosing protocols are in use, but the optimal dose is currently still unknown. A standardized bolus of 5 000 IU heparin is most frequently used by vascular surgeons and interventional radiologists. The activated clotting time (ACT) is an established method to measure the level of anticoagulation, but has, until now, not gained widespread use in NCAP. The purpose of this study was to evaluate the anticoagulant effect during NCAP of a standardized bolus of 5 000 IU heparin by measuring the ACT. Methods: In this prospective study, 190 patients undergoing NCAP were enrolled between December 2016 and September 2018. The ACT was measured during open and endovascular/hybrid procedures. All patients received a standardized bolus of 5 000 IU heparin. The ACT was measured by the Hemostasis Management System Plus (HMS Plus, Medtronic®), before, 5 minutes after administration of heparin, and every 30 minutes thereafter. The primary outcome was periprocedural ACT values measured. Secondary outcomes were ATEC and hemorrhagic complications. Results: A large individual patient variability in the response to heparin was found. The mean baseline ACT in all patients was 129 ± 18 s., and the mean ACT 5 minutes after the initial bolus of heparin was 191 ± 36 s. After the initial dose of 5 000 IU heparin 60 (33%) and 10 (6%) patients reached an ACT of 200 and 250 s., respectively. Despite the use of heparin, ATEC occurred in 17 patients (9%). The lowest number of ATEC occurred in the group of patients with an ACT between 200 and 250 s. Conclusions: A standardized bolus of 5 000 IU heparin does not lead to adequate and safe heparinization in non-cardiac arterial procedures. Patient response to heparin shows a large individual variability. Therefore, routine ACT measurements are necessary to ascertain adequate anticoagulation. Further research is needed to investigate if heparin dosing based on the ACT could result in less arterial thromboembolic complications, without increasing hemorrhagic complications.