Of 568 nonpalpable breast lesions detected by screening mammography 242 proved to be noncalcified masses; biopsy showed 165 to be benign and 77 malignant. The lesions were evaluated with respect to contour, radiographic density and size. Of the malignant lesions 90% were infiltrating carcinomas, 88% of which had an indistinct contour and were of moderate to high density. Of the benign lesions 40% of those with an indistinct contour demonstrated high radiographic density. Low-density, distinct masses were all benign. There was no correlation between the size of the lesion and whether it was malignant.