We reviewed the radiographs of patients undergoing negative breast biopsies on the basis of false positive mammograms. In a small number of these patients, the radiologic error was attributable to the summation effect of superimposed focal areas of normal or dysplastic breast tissue giving the false impression of a mass. This summation effect more frequently results in an equivocal radiologic impression, often with a recommendation for short interval repeat mammograms, rather than an actual biopsy. This false positive mammographic appearance can often be suspected on the basis of a discrepancy in the appearance of the suspicious area on the standard lateral and craniocaudad views. In a prospective study we identified women in whom suspicious mammograms were felt to possibly relate to this superimposition effect, and in this small selected group of patients an additional oblique projection was obtained. The oblique view frequently enabled the initially equivocal abnormality to be definitely assessed.