Surgical specimens, from 78 patients operated for impalpable breast lesions containing microcalcifications detected on mammographs (breast cancer screening program) were systematically examined by a pathologist during the surgical intervention. The per-operative procedures encompassed (i) X-raying of the unfixed specimens to ascertain that the latter did enclose the screened lesion, (ii) slicing and subsequent X-raying of the specimens slices in order to locate the microcalcifications, (iii) histological evaluation of areas containing the microcalcifications on frozen sections. Thirty-two of the lesions were histologically benign, 55% were malignant and 13% borderline. Forty of the carcinomas were in situ and 60% were invasive. The per-operative histological diagnosis was correct in 65% of the case, erroneous in 10% and uncertain in 25%. Malignancy was never overscored. In 65% of the carcinomas diagnosed during the surgical intervention, the in sano margins excision and axillary lymph node removal could be performed (one stage surgical treatment). These results suggest that X-raying of sliced specimens and histological evaluation during the surgical intervention ascertains that lesions screened by mammographs are effectively and completely removed, and that they are precisely and extensively histologically examined after being identified and located.