The pathology of nonpalpable breast cancer. A principal goal of mammographic screening is the early detection of breast cancer. We reviewed records of 125 women who were referred because of nonpalpable, suspicious abnormalities on mammogram, which subsequently proved to be cancer, requiring mammographic localization biopsy and subsequent surgery for therapy. We found that 72 (57.6%) had invasive tumors, 15 (12%) showed evidence of microinvasion and 38 (30.4%) were noninvasive. A total of 115 patients had lymphadenectomy as part of their definitive surgery. Nine (12.7%) of the patients with infiltrating tumors had between one and 10 malignant nodes on histologic section. None of the patients with noninvasive or microinvasive tumors were found to have involved nodes. The mammographic abnormalities which led to biopsy in our series were: calcifications in 74 (59.2%) patients, mass lesions in 39 (31.2%), mass lesions with calcifications in 11 (8.8%), and asymmetry in one (0.8%). Of the nine patients with nodal metastases, seven (77.8%) had a mass with or without calcifications as the indication for biopsy. Increasing tumor size was found to correlate with invasive tumors on histopathologic examination and the incidence of lymph node metastases. Thirty-seven (54.4%) of the patients with infiltrating tumors had a tumor size greater than 1 cm. Further, seven (77.8%) of the nine lymph node positive patients had tumors between 1 and 3 cm in size. Of note, however, is that two (22.2%) patients with microscopic tumors had involved nodes. The 4-year actuarial survival in patients with infiltrating tumors was 85.2 per cent, while that for patients with noninvasive or microinvasive tumors was 100 per cent (median follow-up of 20 months). 