Introduction: Most common site of dentigerous cyst is mandibular or maxillary third molar or a maxillary canine. The patient typically has no pain or discomfort, unless infected. In the present paper, we report the case of a dentigerous cyst associated with an impacted mandibular canine. This cyst was missed initially due to incomplete root canal treatment associated with the nonvital mandibular anterior tooth. The present case of a cystic lesion can create a dilemma between radicular and dentigerous cyst. Aim: we present a case of an infected dentigerous cyst associated with a non -vital mandibular anterior and over-retained primary canine. Case Report: Present case of a cystic lesion can create a dilemma between radicular and dentigerous cyst. A 23 -year old male patient reported to our dental institute with a chief complaint of pain in the lower front region of the jaw for 10 days. Discussion: In the present study, CBCT was helpful in the detection of the relation of impacted canine with the adjacent vital structure which was not appreciated by OPG. Histology showed the connective tissue with dense chronic inflammatory cell infiltrate, aspirated fluid contained red blood cells suggesting that periapical inflammation from non -vital mandibular anterior spread to dentigerous cyst and caused infected dentigerous cyst. Conclusion: One year follow-up was taken healing was seen, there was no evidence of any recurrence after 1 year.