Dentigerous cysts (DCs) are one of the most common cysts in the oral and maxillofacial region, and they are often discovered by chance in young people. The methods of treatment differ according to the size they reach, but the prognosis and results of the treatment are generally good. DCs are often associated with impacted teeth, especially mandibular third molars and maxillary canines, and they are usually discovered when they reach large sizes or get infected after they have caused great absorption and destruction of the surrounding cortical bone and displacement in adjacent teeth. This case report expresses the importance of conservative treatment of large oral cysts (by marsupialization) in the preservation of jaw bones, in a young female child of 12 years, with a DC associated with an impacted second permanent lower molar. This cyst occupied nearly half of the mandible with the danger of causing more harm to the jaw. In brief, marsupialization is a very effective method of treatment for DCs, especially those that reach very large volumes.