A prospective study (protocol SG 89-150) was undertaken to determine the role of mammographically guided fine-needle aspirations in the diagnosis and subsequent surgical treatment of nonpalpable, mammographically detected breast cancers. During this study, once a diagnosis of cancer based on mammographically guided fine-needle aspiration was established, a wide segmental excision was performed to attempt to eradicate local disease. Surgical margins free of tumor were obtained in all cases. Total excision of these small lesions permitted in-depth histopathologic evaluation of the specimens. This led to the discovery that even the earliest detectable breast cancers may have extensive involvement of the surrounding breast tissue, which is vital information for planning complete therapy for the patient with breast cancer.