[en] The purpose of this study was to compare mammography and breast sonography in detection of breast cancer and to suggest reasonable guideline of breast imaging in breast cancer patients under 30 years of age in whom breast cancer shows different clinicopathologic characteristics compared with breast cancer in older women. Authors reviewed medical records of 27 patients under 30 years of age with pathologically-proven breast cancer. Age, family history, physical examination findings, indications for breast sonography were reviewed. Cases in whom breast cancer lesion is detectable and cases in whom not detectable using mammography or breast sonography were reviewed. And then, authors evaluated the usefulness of each method and reasons for nonvisualization of lesion on mammography. Among 27 patients, 25 patients had palpable breast mass as indication of mammography and breast sonography. Cancer lesions were detectable in 16 of 25 patients (64%) on mammography and 24 of 25 patients (96%) on breast ultrasonography. Reasons for nonvisualization of cancer lesions on mammography were dense breast with nodular parenchyma pattern and minimal breast change of ductal carcinoma in situ. In breast cancer patients under 30 years of age who have palpable breast mass as a initial, and main clinical problem, breast ultrasonography is superior to mammography in detecting and diagnosing breast cancer. We suggest that guidelines can avoid unnecessary mammography in these patients