Context.-A review of amended pathology reports provides valuable information regarding defects in the surgical pathology process.Objective.-To review amended breast pathology reports with emphasis placed on interpretative errors and their mechanisms of detection.Design.-All amended pathology reports for breast surgical specimens for a 5-year period at a large academic medical center were retrospectively identified and classified based on an established taxonomy.Results.-Of 12 228 breast pathology reports, 122 amended reports were identified. Most (88 cases; 72%) amendments were due to noninterpretative errors, including 58 report defects, 12 misidentifications, and 3 specimen defects. A few (34 cases; 27.9%) were classified as misinterpretations, including 14 major diagnostic changes (11.5% of all amendments). Among major changes, there were cases of missed microinvasion or small foci of invasion, missed micrometastasis, atypical ductal hyperplasia overcalled as ductal carcinoma in situ, ductal carcinoma in situ involving sclerosing adenosis mistaken for invasive carcinoma, lymphoma mistaken for invasive carcinoma, and amyloidosis misdiagnosed as fat necrosis. Nine major changes were detected at interpretation of receptor studies and were not associated with clinical consequences. Three cases were associated with clinical consequences, and of note, the same pathologist interpreted the corresponding receptor studies.Conclusions.-Review of amended reports was a useful method for identifying error frequencies, types, and methods of detection. Any time that a case is revisited for ancillary studies or other reasons, it is an opportunity for the surgical pathologist to reconsider one's own or another's diagnosis.(Arch Pathol Lab Med. 2017;141:260-266; doi: 10.5858/ arpa.2016-0018-OA) R evised pathology reports can be divided into 2 broad categories: amendments and addenda. An amendment is a change in the information of a pathology report after the report has been finalized and released and requires unsigning the original report. In contrast, an addendum represents additional information added to an unchanged original report. An amendment allows editing of the report text, including diagnostic fields, and therefore, requires adequate documentation of all changes and notifications to referring clinicians. It serves as a record of errors in specimen collection, processing, interpretation, and reporting.A review of amended surgical pathology reports is a valuable tool to identify defects in the surgical pathology process and to promote quality assurance and improvement. Meier and colleagues 1 developed and validated a ''taxonomy of defects'' to approach the review of amended reports in a reproducible manner across different practice settings and across different studies in the literature. According to this approach, amended reports are classified by type of error or ''defect'' into 4 categories-misinterpretations, misidentifications, specimen defects, and report defects-and defect rates and fractions are quality measures...