Purposes: The purpose of this study were to compare mammography and breast sonography in detection of breastcancer and to suggest reasonable guideline of breast imaging in breast cancer patients under 30 years of age inwhom breast cancer shows different clinicopathologic characteristics compared with breast cancer in older women.Materials and Methods: Authors reviewed medical records of 27 patients under 30 years of age withpathologically-proven breast cancer. Age, family history, physical examination findings, indications for breastsonography were reviewed. Cases in whom breast cancer lesion is detectable and cases in whom not detectable usingmammography or breast sonography were reviewed. And then, authors evaluated the usefulness of each method andreasons for nonvisulaization of lesion on mammography. Results: Among 27 patients, 25 patients had palpable breastmass 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 cancerlesions on mammography were dense breast with nodular parenchyma pattern and minimal breast change of ductalcarcinoma in situ. Conclusion: In breast cancer patients under 30 years of age who have palpable breast mass as ainitial, and main clinical problem, breast ultrasonography is superior to mammography in detecting and diagnosingbreast cancer. We suggest that guidelines can avoid unnecessary mammography in these patients.