Purpose : We analysed CT findings of thyroid mass to determine the difference between a benign and a malignant mass and to evaluate the differential findings, if any. Materials and Methods : The subjects were 87 cases with a pathologically proven thyroid mass (malignancy in 66 cases, benign mass in 21 cases). CT findings were retrospectively analysed. For the primary masses, bilaterality, size, margin, attenuation of the mass, spotty portion with distinct high attenuation(which may suggest calcification), necrosis, cystic change, solid portion within the cyst, and invasion of adjacent structures were evaluated. For the lymph nodes, size, high attenuated spotty portion, necrosis, cystic change, and solid portion within the cyst were evaluated. CT findings of thyroid masses and lymph nodes were evaluated in order to determine whether these were benign or malignant. Statistical analysis was performed using the Mann-Whitney U-Wilcoxon rank sum test. Results : In malignant masses, compared with benign, an indistinct margin of the mass(48% vs 19%), invasion of adjacent structures(53% vs 0%), and associated lymph node enlargement(50% vs 0%) were more frequent. With regard to bilaterality, size, attenuation, high-attenuated spotty portion(which may suggest calcification), necrosis, cystic change, and solid portion within the cyst, there was no significant difference between benign andmalignant masses. In masses of the former type, enlarged lymph node or invasion of adjacent structure were not seen at all. When the papillary solid portion within the cystic mass was additionally evaluated, papillany carcinoma was the most common finding(77% 14/18). Conclusion : General findings of malignancy such as margin, invasion of the mass, and lymph node enlargement are of help in the differential diagnosis of a malignant mass. High attenuated spotty portion, which may suggest calcification within the mass, or size of the mass are non-specific findings, and are not helpful in differential diagnosis. The papillary solid portion within the cyst of the mass could suggest papillary carcinoma as a first possibility and could be helpful in differential diagnosis.