Ameloblastomas are bening intraosseous lesions affecting maxillary jaws. They originate from the epithelium involved with the formation of teeth such as enamel, odontogenic rests of Malassez, reduced enamel epithelium and odontogenic cyst lining. Thus, the hypothesis of transformation of a dentigerous cyst into an ameloblastoma was suggested in the literature. These lesions are locally invasive and can grow to infiltrate soft tissues. The risk of recurrence after surgical treatment is important and a long term survey is indicated. The aim of this paper is to demonstrate the diagnostic dilemma in front of a well-defined radiolucent lesion of the mandibule associated to an impacted wisdom tooth and the diffuclties on therapeutic decision. A healthy 34-year-old man referred to our dentistry department complaining about the recurrence of inflammatory episodes associated with tooth number 48. The radiological examination showed a large well limited radiolucent lesion related to tooth number 48, situated in the ramus region. The patient underwent tooth extraction and cystic enucleation. The anatomopathological examination confirmed the diagnosis of a follicular ameloblastoma. Nevertheless, the cystic wall showed both a detigerous cystic portion in one part and a follicular ameloblastoma segment in another part, confirming the hypothesis of ameloblastoma transformation of a detnigerous cyst.