BACKGROUND: Early detection of breast cancer is important because it can reduce morbidity and mortality. Mammography is widely used for clinical and screening purposes and was contributed to the early detection of breast cancer, even tumors less than 1 cm in size and microcalcifications without lumps. METHODS: We retrospectively analyzed the clinical records of 340 patients with clinically evident breast cancer and 55 patients with clinically occult breast cancer who were treated at the Department of Surgery, Samsung Medical Center. Preoperative mammography, ultrasonography, and cytology, as well as operative method and postoperative pathology, were compared to evaluate the clinical features of clinically occult cancer. Preoperative marking was performed in suspected lesions by means of hookwire insertion or, in case of breast conservation surgery, by means of tattooing. RESULTS: Based on mammography, malignancy was suspected in 39 out of 42 cases (92.8%). Micro calcifications were seen on the mammography in 19 patients (45.2%), a mass in 17 (40.5%), a mass with microcalcifications in 4 (9.5%), and an asymmetric increased density in 2 (4.8%). Microcalcifications were major findings in comparing with clinically evident cases (16.9%) (p=0.001). Ultrasonography detected a malignancy in 10 patients (76.9%) and a core biopsy under ultrasonography was performed in 9 of them. Among the 39 patients with a mammographically suspected malignancy, a stereotactic core biopsy was performed in 12 patients, an ultrasonography-guided core biopsy in 9, fine needle aspiration cytology in 10, and localization & excisional biopsy in 3; the remaining 5 cases underwent surgery without additional evaluation. Breast conservation surgery was performed in 23 patients (41.8%) and modified radical mastectomy in 31 (56.4%). In the breast conservation surgery, 19 patients were localized by using mammographic needle localization and 4 patients by using ultrasonographically tattooing preoperatively. A ductal carcinoma in situ (DCIS) was more frequently found in the group with clinically occult breast cancer (10 patients, 18.2%) than in the group with clinically evident breast cancer (2.6%, p<0.001). Postoperative staging was earlier in patient with clinically occult cancers than in those with clinically presenting masses (p<0.001). CONCLUSION: By using mammography and ultrasonography, breast cancer can be diagnosed before it becomes palpable, and thus it can be treated with conservative surgical procedures.