OBJECTIVE: Assessing the biomechanical behaviour of the aortic wall in vivo can potentially improve the diagnosis and prognosis of patients with abdominal aortic aneurysms (AAA). With ultrasound, AAA wall stiffness can be estimated as the diameter change in response to blood pressure. However, this measurement has shown limited reproducibility, possibly influenced by the unknown effects of the highly variable ultrasound probe pressure. This study addressed this gap by analyzing time-resolved ultrasound sequences from AAA patients. METHOD: Two-dimensional ultrasound sequences were acquired from AAA patients, alternatively applying light and firm probe pressure. Diameter variations and stiffness were evaluated and compared. An in silico simulation was performed to support the in vivo observations. RESULTS: It was found that half of the AAAs were particularly responsive to the probe pressure: in these patients, the cyclic diameter variation between diastole and systole changed from 1% at light probe pressure to 5% at firm probe pressure and the estimated stiffness decreases by a factor of 6.3. Conversely, the other half of the patients were less responsive: diameter variations increased marginally and stiffness decreased by a factor of 1.5 when transitioning from light to firm probe pressure. Through numerical simulations, we were able to show that the rather counter-intuitive stiffness decrease observed in the responsive cohort is actually consistent with a biomechanical model. CONCLUSION: We conclude that probe pressure has an important effect on the estimation of AAA wall stiffness. Ultimately, our findings suggest potential implications for AAA mechanical characterization via ultrasound imaging.