Five hundred fifty-seven biopsies were performed for clinically occult mammary lesions, detected by mammography as clustered calcifications or nonpalpable masses within the breast. One hundred seventy-five cancers were demonstrated within this group, including 106 invasive carcinomas, 10 microinvasive carcinomas, 45 in situ ductal carcinomas, and 14 lobular carcinomas in situ (lobular neoplasia). No patient with in situ or microinvasive carcinoma had evidence of axillary node metastases in 33 specimens studied. However, a disturbingly high proportion of those patients with invasive carcinomas, approximately 35%, had histologically confirmed axillary node metastases, despite the small size of the primary tumors. These observations suggest that the use of the term "minimal" cancer is misleading when applied to invasive carcinoma. Staging systems for breast cancer have been imprecise when referring to nonpalpable lesions. Cancers detected as clustered calcifications only or as areas of parenchymal distortion without an accompanying mass are properly considered as T-0 cancers, with a suggested T-0(m) to indicate that the lesion was detected by mammography. However, when the mammogram indicates the presence of a mass that proves to be malignant, although the clinical examination may have been negative, the cancer should be staged according to the size of the mass on the mammogram, with the notation that it was detected by mammography, e.g., T-1(m), T-2(m), etc. The incidence of axillary node metastases even in these so-called occult cancers is significant, so that recommendations for treatment for any invasive cancer, regardless of its size, must take these observations into account. Similarly, the incidence of multifocal sites of cancer within the breast, even in the noninvasive cancers encountered, must be remembered when treatment is suggested.