PURPOSE: This study is aim to know detection rate of masses on mammogram, according to nature and size of masses shown on breast sonogram. MATERIALS AND METHODS: The 176 masses were found on breast sonogram of patient that visited our hospital during recent 1 year. We analyzed retrospectively detection rate of masses on mammogram, and difference of detection rate according to nature (cystic, solid), and size (more than 1 cm, less than 1 cm) of masses which were showed on breast sonograrn. RESULTS: 176 masses were found on breast sonogram and only 66 masses of them (37.5 %) were found on mammogram. 135 solid masses were found on breast sonogram and and only 56 masses of them (41.5 %) were found on mammogram and 43 solid masses were more than 1 cm in size. 41 cystic masses were found on breast sonogram, and only 10 masses of them (24.4%) were found on mammogram. The remained 31 masses were not found on mammogram and 24 masses of them were less than 1 cm in size. There is statistically signifcant difference between detection rate of solid and cystic mass, and detection rate of mass more than 1 cm and less than 1 cm (Chi-square teat: p<0.05). CONCLUSION: The detection rate of mass was low on mammogram, as compared with breast sonogram, because dense breast concealed the masses on mammogram. Most of solid masses more than 1 cm in size were found on mammogram in the cases without dense breast, but breast sonogram should be always performed in the cases with dense breast on mammogram.