A new apparatus has been used for the localization of 46 nonpalpable mammary lesions seen in either of two radiological projections. This device compresses and immobilizes the breast independently of the mammography equipment and permits a three-dimensional localization of the lesion by using the technique of X-ray angulation. The results demonstrate the efficiency of the procedure. Transfixation of the lesions was established in 30 cases (65.2%), the guidewire passed very close to the lesion in 14 cases (30.4%), and in 2 cases (4.4%) the guidewire was seen to be less than 10 mm away from the lesion. Correct preoperative localization of the lesions permitted a correct diagnosis with minimal biopsies.