Purpose : We evaluated the MR and CT findings of the masticator space lesions in order to edentify the differences among the malignant and benign tumors and infectious conditions. Materials and Methods : MR and CT findings in 46 cases with proven masticator space lesions were reviewed retrospectively. We analysed the involvement of masticator muscles, adjacent spaces, orbit and intracranium, homogeneity, necrosis, cystic changes, growth patterns, calcifications, enhancement patterns, MR signal intensity, and CT attenuation. Results : Among the 29 cases of malignant tumors, seben cases were mandibular tumors including four chondrosarcomas, and 22 cases were extramandibular tumors. Malignant tumors of mandibular origin showed large masses with severs bone destruction and epicenter of mandible. Extramandiblular malignant tumors showed the epicenter out of the mandible and less severe bone destrucion than mandibular tumors. Among the nine benign tumors, four cases were amelblastomas which showed the well-defined masses and the expansion of the mandible, and four cases were extramandibular tumors which showed well-marginated extramandibular masses with no bone destruction. Among the eitht infectious conditions, five cases were mandibular osteomyelitis with or without abseess formations, and the other three cases were infections from adjacent soft tissue or limited to the soft tissue. Conclusion : By careful observations of growth patterns, involvement of the masticator and adjacent spaces, bone changes, and epicenter of the lesions, one can discriminate a mandibular lesion from an extramandibularlesion. With this approach, it is thought to be easier to suggest a dignosis among a wide spectrum of masticatorlesions.