The purpose of this study was to obtain some informations for the radiographic differential diagnosis between dentigerous cysts and unicystic ameloblastomas in the mandible. The authors observed and compared the clinico-radiographic features of 38 cases of dentigerous cyst and 32 cases of unicystic ameloblastoma associated with impacted mandibular molar. The obtained results were as follows: Dentigerous cysts occurred the most frequently in the 3rd decade, but unicystic ameloblastomas in the 2nd decade, and both lesions occurred with slight predilection in males. Average of lesional size of unicystic ameloblastomas was larger than that of dentigerous cysts, and lesions of over 25㎠ were only in unicystic ameloblastomas. Cortical thinning and expansion were more frequently observed in unicystic ameloblastomas at 72.9% than in dentigerous cysts at 15.8%. Dentigerous cysts showed smooth border at 89.5%, but unicystic ameloblastomas showed smooth border at 53.1% and scalloped border at 46.9%. Dentigerous cysts showed well-defined outline at 81.6%, but unicystic ameloblastomas showed well-defined outline at 53.1% and moderate-defined outline at 46.9%. In both lesions, the mandibular 3rd molar was the most frequent causative tooth. Average of distance between the cemento-enamel junction and lesional wall attachment of the causative tooth was longer in unicystic ameloblastomas than in dentigerous cysts. Severe displacement of causative tooth was more frequent in unicystic ameloblastomas at 62.5% than in dentigerous cysts at 23.7%. Dentigerous cysts showed homogeneous lesional radiolucency at 89.5%, but unicystic ameloblastomas showed inhomogeneous lesional radiolucency at 53.1%. Root resorption of adjacent tooth and displacement of mandibular canal were more frequent in unicystic ameloblastomas at 65.2% and 61.5% than in dentigerous cysts at 15.8% and 38.1% respectively.