BACKGROUND AND AIMS: Antiphospholipid Syndrome (APS) is often complicated by ischemic vascular events. Vitamin K Antagonists (VKAs) reduce the risk of recurrent thrombosis. Quality of VKAs treatment, as assessed by the Time in Therapeutic Range (TTR), has never been investigated in APS patients. METHODS: We performed a prospective observational study including 30 APS and 30 Atrial Fibrillation (AF) patients balanced by age and gender. All patients were treated with VKAs (INR target 2.5), and TTR was calculated. RESULTS: Median TTR of APS was 53.5% vs. 68% of AF patients (p = 0.001). A multivariable linear regression analysis confirmed that the presence of APS (vs. AF) was independently associated with a worse TTR (B: -14.067, 95% Confidence Interval -25.868/-2.266, p = 0.020). The weekly dosage of VKAs was significantly higher in APS than AF patients. CONCLUSIONS: APS patients disclose a lower quality of anticoagulation compared to those with AF, requiring higher doses of VKAs. The efficacy of non-vitamin K oral anticoagulants in this high-risk patients should be tested.