CT was done on 20 cases of proven histologically malignant tumors of maxillary sinus for 2 years from July 1982 to June 1984 at Han Yang University Hospital. The results were following : 1) Sex ratio of male to female was 2.3 : 1. 2) Age distribution was from 20 to 67 year old. Maxillary cancer was most frequently occured in the 6th decade. 3) Squamous cell carcinoma was most common as 11 cases histopathologically. Others were 5 cases of malignant lymphoma, 2 cases of adenoid cystic carcinoma, 1 case of malignant melanoma and 1 case of metastatic carcinoma from parotid gland. 4) CT findings were sinus opacification, soft tissue mass, and bone destruction in all cases, fat plane obliteration in 13 cases, bone erosion and displacement in 9 cases and osteosclerosis in 7 cases. 5) Advantages of CT in malignant maxillary sinus tumors comparing with conventional radiologic technique were evaluation of extension to the orbit, infratemporal fossa, pterygopalatine fossa, nasopharynx, pterygoid fossa and intracranial fossa, staging of maxillary sinus tumors and assessing operability and adequacy of radiotherapy fields. 6) Contrast enhancement was of little value in the delineation of maxillary sinus tumors. But intracranal extension of tumor was clearly defined as a result of enhancement. 7) It was impossible to differentiate soft tissue masses of malignant origin from benign origin by available CT technique. Biopsy was the only reliable method for diagnosing malignancy.