Three hundred and fifty-three women underwent 358 biopsies of the breast for nonpalpable mammographic lesions during a five year period. Cancer was identified in 95 (27 per cent). Mammographic findings in the patients with cancer were calcifications only (54 per cent), a mass (27 per cent), a mass with calcifications (15 per cent), an asymmetric distortion (1 per cent) and an asymmetric distortion with calcifications (3 per cent). Cancer was identified in 29 per cent of the biopsies done for calcifications, 20 per cent of those done for a mass with calcifications, 6 per cent of those done for an asymmetric distortion and 38 per cent of those done for an asymmetric distortion with calcifications. Forty-nine per cent of the mammographically suspicious calcifications were cancer, while 100 per cent of the indeterminate calcifications were benign. Forty-five per cent of the spiculated masses were cancerous, while only 3 per cent of the circumscribed masses were malignant. For those patients with carcinoma undergoing axillary lymph node dissection, 13 per cent had one or more positive nodes. A strategy for increasing the effectiveness of mammographically guided biopsies of the breast is presented.