Mammograms, including microfocus spot magnification views, were obtained before reexcision of the tumor-ectomy site in 43 women with breast carcinoma. These studies were prospectively evaluated by radiologists experienced in breast imaging. All women had mammographically evident microcalcifications associated with their original tumors. In all, tumor was at or near the margin of resection or the status of margins was unknown, necessitating reexcision of the tumorectomy site. Of 29 women with residual microcalifications, 20 had residual tumor. The positive predictive value of residual microcalcifications was 0.69. The positive predictive value was greatest (0.90) in women with ductal carcinoma in situ, when more than five microcalifications were present. Residual microcalifications not due to carcinoma were secondary to sclerosing adenosis, fat necrosis, and foreign body granuloma. Of the 13 cases in which no residual calcifications were left after tumorectomy, residual tumor was found in four. The negative predictive value of the absence of calcifications for total excision of tumor was 0.64 for all tumor types and was 1.0 for the noncomedo subtype of ductal carcinoma in situ.