A radicular cyst (Periapical cyst, Apical periodontal cyst, Dental cyst) is a true cyst that occurs in association with the root of non vital tooth. It is most commonly occurring cyst in the oral region and found at the apices of teeth, however they may also be found on the lateral aspects of the roots in relation to lateral accessory canals. They arises from the epithelial remnants in periodontal ligament as a result of stimulus from periapical infection following pulp necrosis. This case highlights the Cone-Beam CT- guided diagnosis of a large infected radicular cyst in a 48-year old female patient. The diagnosis was confirmed histopathologic ally, following which marsupialisation was carried out.