Purpose: To clarify the clinical and radiologic parameters that can be used to differentiate odontogenic keratocyst and ameloblastoma Materials and Methods: The records of 46 patients of ameloblastoma and 48 patients of odontogenic keratocyst at the Yonsei University Dental Hospital during the period of 1979 to 1995 were retrospectively reviewed. As a possible means for differentiating between the odontogenic keratocyst and ameloblastoma, the clinical parameters and the radiologic parameters were evaluated. Results: In the clinical parameters, there was no significant difference in age, sex, and sign and symptoms (p>0.05). In the radiologic parameters, there was significant difference in site, shape of the lesion, and external root resorption of adjacent teeth (p<0.05). Conclusion: The site, shape of the lesion, and external root resorption of adjacent teeth can be the parameters to differentiate odontogenic keratocyst and ameloblastoma, but a definite differentiation of these two lesions needs a more specialized imaging modality.