Several studies have demonstrated that mammographic screening of asymptomatic women results in a lower mortality rate where breast cancer is concerned. Often, screening mammograms reveal a nonpalpable radiographic abnormality and the diagnosis must be determined by an excisional biopsy after radiographic needle localization. The mammographic features associated with 179 nonpalpable breast abnormalities biopsied after radiographic needle localization were carefully characterized. There were 41 carcinomas (23%) in the series. The aim of this study was to determine which radiographic findings, if any, strongly portend the presence of either a malignant or benign lesion. Mammographic features that were commonly associated with malignancy include a change from a previous mammogram, a distortion of the surrounding architecture, the association of a soft tissue density and calcifications, and the presence of more than ten calcifications in the lesion. The radiographic abnormalities which were more commonly associated with benign disease include well-defined densities without calcifications, asymmetric densities without calcifications, and abnormalities consisting solely of a focus of mammographic calcifications that have fewer than ten concretions. The incidence of malignancy in lesions having these mammographic characteristics was only 5.5%. On the basis of these results alone, no firm threshold for biopsy can be recommended. The risks of deferring biopsy until there is worsening of the mammographic image remains to be determined.