A prospective study was performed to assess the reliability of sonographic mammography in mass screening for breast lesions and diagnosis of small, presumably early, breast cancers. 1350 women underwent a sonomammographic examination of the breasts. Sonomammographic images were described in terms of their elementary constituents. Final diagnosis was obtained through cytological, and, when indicated, histological analysis. This ultimately led to the diagnosis of 13 breast cancers. Statistical analysis of the elementary sonomammographic image data using a logistic regression found two criteria to be highly associated with cancer. These were: (1) jagged aspect of the margins extending to the anterior or posterior walls; (2) major axis of the lesion not parallel to the skin surface. When combined, these two criteria have a 100% sensitivity and 99.6% specificity. The conclusions of the present study were validated for the screening of young women and may be applied to the screening of small lesions. However, these conclusions should be restricted to tumors 5 mm or more in diameter and of the common histological type, i.e. the infiltrating ductal carcinomas. Within these limitations, sonographic mammography can be considered a reliable technique for mass screening for breast cancer.