To determine the likelihood of malignant disease for mammographically detected, nonpalpable breast lesions, we defined 11 morphologic categories and retrospectively reviewed the mammograms in 859 cases in which biopsy was performed after a wire localization procedure at our institution during 1989 and 1990. Within each category, the total number of lesions and the percentage of cases with a surgical pathologic diagnosis of malignant involvement were as follows: benign calcification, 25 (0% malignant); indeterminate calcification, 200 (22%); malignant calcification, 39 (92%); smooth mass, 84 (1%); irregular mass, 337 (40%); architectural distortion, 45 (47%); asymmetric breast tissue, 37 (3%, or 1 case of asymmetrically prominent ducts); smooth mass with calcification, 3 (0%); irregular mass with calcification, 68 (66%); architectural distortion with calcification, 14 (57%); and asymmetric breast tissue with calcification, 7 (29%). The overall rate of malignant involvement for the 859 cases was 34%. If follow-up examinations rather than biopsies had been done for the lesions categorized as benign calcification, smooth mass, smooth mass with calcification, and asymmetric breast tissue (excluding asymmetrically prominent ducts), the overall positive predictive value would have increased from 34 to 41%, and 148 biopsies would have been deferred (17% of all biopsies). If morphologic criteria are applied to the evaluation of mammographically detected, nonpalpable lesions, the rate of malignant disease at biopsy may reach 40%. This rate correlates with that in recent large series.