Objective: Low-molecular-weight heparins are frequently used to prevent venous thromboembolism. Vasopressor therapy may be associated with inadequate anti-factor Xa activity, thereby increasing the risk of venous thromboembolism. We aimed to assess the association between anti-factor Xa activity and norepinephrine dose in intensive care unit (ICU) patients treated with subcutaneous dalteparin for venous thromboembolism prophylaxis. Materials and methods: This was a prospective observational pilot study in adult ICU patients treated with dalteparin 5,000 IU subcutaneously once daily and norepinephrine > 0.25 µg/kg/min. Peak anti-factor Xa activity was monitored and dalteparin doses were adjusted following a predefined dose algorithm. Results: From November 2016 to April 2018, 32 patients... were included. No correlation was found between norepinephrine dose and anti-factor Xa activity (r = –0.01, 95% confidence interval = –0.47 – 0.27, p = 0.57). Furthermore, following dalteparin 5,000 IU once daily, 28% of the patients showed anti-factor Xa activity < 0.10 IU/mL. Higher body mass index (BMI) (p < 0.001) but not patients’ norepinephrine dose, age, or serum creatinine were risk factors for anti-factor Xa activity < 0.10 IU/mL. Dose increments to 7,500 IU once daily resulted in anti-factor Xa activity ≥ 0.10 IU/mL in all 5 patients (p = 0.043). Conclusion: In this cohort of ICU patients, no association was found between norepinephrine dose and anti-factor Xa activity following subcutaneous dalteparin 5,000-IU administration once daily. Furthermore, nearly one-third of the patients showed anti-factor Xa activity below the target concentration for venous thromboembolism prophylaxis. Higher BMI was an independent risk factor for reduced anti-Xa activity.