Background and aim of the study. There is limited data regarding the duration of endothelialization following paravalvular leak closure. We aimed to observe the endothelialization process in 2 patients who underwent surgery 6 and 16 months after failed percutaneous mitral paravalvular leak closure, respectively. Methods. Two-dimensional transesophageal echocardiography (2D-TEE) and real-time 3-dimensional transesophageal echocardiography (RT-3D TEE) were utilized to demonstrate mitral paravalvular leaks. The status of endothelialization was explored in the surgery. Results. Two patients underwent percutaneous closure of mitral paravalvular leaks both with 2 occluder devices. The first patient was admitted with dyspnea 6 months later. RT-3D TEE demonstrated a defect around the proximal part of one of the occluder devices. The residual mitral regurgitation was considered moderate to severe by 2D TEE and RT-3D TEE. The patient was referred to surgery in which failed endothelialization of both devices was observed. In the second patient, 2 occluder devices were implanted. He underwent surgery at 16 months due to progressive increase in the severity of mitral regurgitation, which disclosed partially endothelialized closure device. Conclusion. These cases suggest that endothelialization of closure devices may be significantly delayed or even absent for a long time following implantation.