In atrial fibrillation (AF), stroke risk, secondary to thrombus formation in the left atrial appendage (LAA), is the most life-threatening condition. Occlusion of the LAA (LAAO) is a novel strategy for stroke prophylaxis. Several devic-es have been developed specifically for LAAO. Unfortunately, the occlusion procedure is associated with major adverse events and careful attention should be paid in patient selection. The aim of this study was the simulation of the fluid dynamics effects of the LAAO to predict patient-specific fluid dynamics changes due to LAAO. Two patient-specific 3D anatomical models of the LA were obtained from CT. The LAA was automatically detected and removed. LAAO with two dif-ferent devices among the most used in clinical practice (AmuletTM and WatchmanTM) was reproduced at the ostium. For each patient, three fluid dynamics simulations were performed using three different LA anatomical models (with LAA, LAAO with WatchmanTM, LAAO with AmuletTM). AF was simulated by employing a random displacement model of small amplitude. Overall, fluid dynamics simulations with the LAA showed different spatial distribution of velocities and higher velocities throughout the cardiac cycle. Importantly, at the mitral valve and at the LAA ostium velocities were higher after LAAO. Similar hemodynamic patterns were observed in the simulations of the two different LAAO and in both patients. However, some differences were noted especially in the number of fluid particles inside the LA after five cardiac cycles, that was higher for the first LAAO device. During atrial systo-le, a more chaotic fluid pattern was observed for the second LAAO device, probably due to its concave shape causing variations in the fluid streamlines direction. Our preliminary results suggest (1) a lower risk of blood clot formation after LAAO; (2) a slightly improved washout effect when the WatchmanTM device is implanted. These results need confirmation in a larger population.