This case describes the treatment and follow-up of a mandibular molar in an 18-year-old female with a periapical cyst. Thus, it becomes important to know which options should we take when faced with a clinical situation that we cannot resolve through conventional methods, and which techniques and approaches we have to achieve treatment success. This case showed the treatment plan and follow-up, by the use of CBCT images, from a previously treated mandibular molar with a large periapical abscess and cystic lesion, in which, the first treatment plan approach was to make the endodontic retreatment. During the chemo-mechanical preparation the presence of permanent intracanal purulent exudate made it impossible to dry the canals, impeding obturation of the root canal system. Due to this clinical situation, a surgical approach was performed with the intention of reduce this permanent exudate and to execute a decompression technique. Clinical findings, periapical radiographs and cone beam computed tomographic, indicated almost complete resolution of the radiolucency, after a one-year follow-up.