Incomplete LAA Occlusion by WATCHMAN Device. Introduction: Transcatheter left atrial appendage (LAA) closure with the WATCHMAN device has become one of the therapeutic options in atrial fibrillation (AF) patients who are at high risk for ischemic stroke. However, the incidence and evolution of incomplete occlusion of the LAA during and after placement of the WATCHMAN device has not been reported. Methods and Results: Fifty-eight consecutive patients who had undergone WATCHMAN device implant were included in the study. Intraprocedural, 45-day and 12-month transesophageal echocardiogram images were reviewed and analyzed. Peridevice gap was noted in 16 (27.6%), 17 (29.3%), and 20 (34.5%) patients across the 3 time points. Intraprocedural gaps are more likely to be persistent until 12 months and become larger in size over time. New gap also occurs during follow-up even if the LAA was completely sealed at implantation. One patient had an ischemic stroke 4.7 months after implant; another patient developed a left atrial thrombus over the device 21.6 months after implant. Both patients had intraprocedural gap and discontinued warfarin therapy after the 45-day evaluation. Conclusion: Incomplete LAA occlusion with a gap between the WATCHMAN device surface and the LAA wall is relatively common. Intraprocedural gaps are more likely to become bigger over time and persist, while new gaps also occur during follow-up. Further studies are warranted to verify whether the presence and persistence of a peridevice gap is associated with increased risk of thromboembolic event in AF patients implanted with a WATCHMAN device. (J Cardiovasc Electrophysiol, Vol. 23, pp. 455-461, May 2012)