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Background: Hypertensive crisis is an extreme phenotype of increased blood pressure that can lead to organs failure and thrombotic complications. Recently, we were able to show an angiotensin II driven FXI-thrombin amplification loop leading to vascular injury in experimental hypertension. Objective: The aim of this study was to evaluate the thrombin generation (TG) in patients with uncontrolled arterial hypertension (AHt). Patients and methods: We prospectively examined 27 patients with uncontrolled AHt at the emergency department of the medical center of Mainz, and 26 age match controls. TG was measured by calibrated automated thrombography (CAT) in platelet rich and platelet poor plasma (PRP/PPP). Results: AHt patients had an increased systolic blood pressure, compared to control patients (182±10.7 versus 134±9.6 mmHg); age, BMI and weight were not different. Unexpectedly, CAT assay performed in PPP showed a decreased of TG in uncontrolled AHt patients (1269±55 versus 1444±51 nM.min) as well as a decrease in the peak of generation. The TG performed in PRP was identical between uncontrolled AHt and control patients (1550±65 versus 1513±58 nM.min), but the peak of generation, as well as the velocity, were increased in uncontrolled AHt patients. In both groups, TG was reduced by blocking the apple 3 domain of FXI, indicating an involvement of the FXI thrombin loop in thrombin generation in PRP of uncontrolled AHt patients. Conclusion: These results point out the important role of platelet overreactivity in hypertension. Monitoring the prothrombotic state of platelets might add to risk stratification of patients with AHt.