PURPOSE: We retrospectively performed this study to evaluate the charac6teristic findings of maxillomandibular ameloblastomas on CT and MR imaging. Materias and Methods:We reviewed histologically proved 12 cases of ameloblastomas, of which 7 cases were postoperative recurrent tumors. one of twelve cases was presumed ameloblastic carcinoma. Eleven cases were examined with CT and 3 cases with MR. RESULTS: The types were solid in 4, unicystic in 4, and mixed in the rest 4. CT and MRI of 11 ameloblastomas showed concentric expansile mass(n=11), cortical bone thinning and focal bone destruction by the tumors(n=9), well-margined, expansile destruction of surrounding sturctures(n=9), focal bulging of the tumors(n=6) and focal poorly-marginated invasion of tissue planes(n=4). Ameloblastic carcinoma showed ill defined irregular margin, aggressive invasion of surrounding structures and hematogeneous lung metastasis. Unerupted teeth or mural nodules were found in unicystic ameloblastomas. All three tumors examined by MRI showed isointensity to muscle on T1 weighted images and slight hyperintensity on T2 weighted images. The wall, septa and solid portions of the tumors were strongly enhanced on MR imaging. There was no difference in CT or MR finding between primary and recurrent tumors. CONCLUSION: Ameloblastomas showed solid, cystic or mixed pattern, and commonly well marginated expansile contour with local aggressiveness. Presence of mural nodules on CT in unicystic ameloblastoma with unerupted tooth was helpful in distinguishing ameloblastoma from dentigerous cyst.