1. A retrospective study of 72 biopsies on the basis of mammographic findings only, indicates that the criteria used for identification of suspicious lesions produce a high yield of positive cases (42 per cent). 2. Clustered calcifications less than 3 mm. in diameter are the most frequent indicators of nonpalpable breast cancer. Six to 15 such calcifications in a small area (square centimeter or less) were associated with cancer in 23 per cent of the cases, and clusters with more than 15 calcifications in a small area indicated cancer in 82 per cent of the patients in this study. 3. Similar calcifications prevent differentiation between sclerosing adenosis, intraductal hyperplasia, papillomatosis, and carcinoma on mammography. 4. Of the criteria for mammographic identification of nonpalpable cancer, clustered calcifications without other changes are the most favorable (earliest?) finding. In this study, 19 cancers were diagnosed on this basis. Of these 14 (74 per cent) were noninvasive, and 18 (95 per cent) were lymph node negative.