Cystic odontoma is a rare entity, which is characterized by the association of a cyst with complex/compound odontoma. The aim of this study was to report the case of a 5-year-old male patient diagnosed previously with Hodgkin's lymphoma and treated successfully with chemotherapy and radiotherapy, who developed a mandibular odontogenic lesion. Physical examination revealed a swelling on the right side of the mandible. Radiographically, a well-defined radiolucent area surrounded by radiopaque material was observed. An incisional biopsy was performed and microscopic analysis showed a cystic lesion consisting of an atrophic epithelium comprising 2-3 cell layers and the absence of inflammation in the cystic capsule. The cyst was decompressed and the lesion was removed after 3 months of follow-up. Microscopic analysis of the surgical specimen showed a cystic hyperplastic epithelium surrounded by an intense chronic inflammatory cell infiltrate, which was in close contact with mineralized tissue resembling dentin and cementum. The final diagnosis was cystic odontoma. Since chemotherapy can affect the growth and development of infant teeth, a relationship between chemotherapy-associated adverse events and cystic odontoma is suggested in the present case.