In recent years, owing to changes in clinical practice, the use of the frozen section to diagnose mammary carcinoma has declined. Malignancy is usually diagnosed by either core needle biopsy or open excisional biopsy of ambulatory patients. The need to consider treatment alternatives precludes the use of frozen section diagnosis immediately before definitive treatment (2). Frozen sections may be appropriate for the confirmation of a cytologic diagnosis of carcinoma or the evaluation of surgical margins. If frozen section is undertaken and any doubt arises about the diagnosis, a final decision should be deferred until processed tissue is available. Difficulties are usually attributable to sampling errors, technical problems, or occasionally true histologic misinterpretation. Errors in interpretation arise particularly with sclerosing lesions (sclerosing adenosis, radial scar, ductal adenoma), epithelial hyperplasia, papillary lesions, and fat necrosis. The diagnosis of in situ carcinoma can also cause problems, not only because it may be difficult to distinguish from benign epithelial hyperplasia but also because stromal invasion cannot always be excluded in the small sample usually examined.