Pathologic discriminants such as lesion size, ratio of infiltrating to intraductal growth, histologic type, histologic grade, percentage tubule formation, lymph vessel invasion, and axillary lymph node metastases were used to analyze 183 breast cancers detected on screening by mammography and physical examination. In general, cancers detected only on mammography should be associated with favorable long-term survival because of their early stage. These lesions had histologic features indicating significant potential for subsequent metastatic spread, which would have occurred had screening been performed by physical examination alone.