The dentigerous cyst is reported to be one of the most common odontogenic cyst, of developmental origin, which is characterized by its association with an impacted tooth. Often assymptomatic, the dentigerous cyst presents as a well-limited unilocular radiolucent image surrounding the crown of an unerupted or impacted tooth. Decompression is a conservative treatment option for dentigerous cysts, used especially in children, to preserve the cyst associated tooth, promote its eruption and to avoid lesions of the mandibular canal. This report presents a mandibular dentigerous cyst associated with the 35 impacted tooth in a 12-year-old girl. The dentigerous cyst was successfully treated by surgical decompression.