CT was done 81 times on 63 proven malignant maxillary sinus tumors for 3 years from February 1979 to May 1982 at Seoul National University Hospital. Pre-treatment CT were 54 and post-treatment CT were 31 on 28 patient. The result were as follows; 1. The most frequent histopathologic diagnosis was 44 cases (69.8%) of squamous cell carcinoma. Others were 5 cases (7.9%) of adenoid cystic carcinoma, 3 cases (4.8%) of olfactory neuroblastoma, 3 cases (4.8%) of malignant lymphoma, 2 cases (3.2%) of melanoma, 2 cases (3.2%) of malignant fibrous histiocytoma, a rhabdomyosarcoma, a basal cell carcinoma, a fibrosarcoma and a metastatic carcionoma from thyroid follicular adenocarcinoma. 2. The CT findings in 54 untreated malignant maxillary sinus tumors were sinus opacification, soft tissue mass, and bone destruction in all cases. Other findings were fat plane obliteration (70%), osteosclerosis (59%), bone erosion and displacement (46%), low densities within soft tissue mass (27%), and air densities within soft soft tissue mass (27%), and air densities within soft tissue mass (13%). 3. The value of pre-treatment CT in malignant maxillary sinus tumors were outlining the disease process especially soft tissue such as orbit, infratemporal fossa, pterygopalatine fossa, nasopharynx, pterygoid fossa and intracranial extension, and CT is the choice of diagnostic modality to determine the prognosis and the therapeutic planning in surgery and/or radiotherapy. 4. Post-treatment CT is also helpful to evaluate the change in tumor size and post-treatment complication.