Purpose : The purpose of our study was to compare the diagnostic accuracy of mammography and breast ultrasonography between fibroadenoma and palpable breast cancer and to evaluate the feasibility of ultrasonography as a primry diagnostic modality to differnetiate between these two tumor types. Materials and Methods : In 36 cases of fibroadenoma and 35 of breast cancer, all palpable and pathologically-proven, the diagnostic accuracy of mammography and ultrasonography was retrospectively analysed. Results : In firbroadenoma cases, the diagnostic accuracy of mammography and ultrasonography was 53% and 80%, respectively, and the difference was statistically significant(p = 0.0162). In cases of breast cancer, the corresponding figures were 74% and 82%, respectively ; the difference was not statistically signigicant(p = 0.55). Overall, the diagnostic accuracy of mammography and ultrasonography was 63% and 82%, respectively, and this difference was statistically significant(p = 0.0164). The total diagnostic accuracy of both studies was 90%, and this was signigicantly different(p = 0.044) from that of ultrasonography(82%). Conclusion : In patients who have clinically palpable breast masses, ultrasonography can be recommended as the primary diagnostic modality, though for other breast lesions, mammography is the recommended primary modality.