Accurate estimation of disease extent and margin status is critical when evaluating partial mastectomy cases because both are predictors of recurrence. No published standards exist for processing specimens involved by invasive carcinoma, presumably because such cases have a gross lesion. We retrospectively studied 100 partial mastectomy cases and concluded that a standardized tissue mapping protocol is needed to ensure adequate pathologic examination even when a gross lesion is present. When mapped and unmapped findings were compared, 17 cases (10 with ductal and 7 with lobular carcinoma) had an increase in carcinoma size, 12 cases (9 with ductal and 3 with lobular carcinoma) had an increase in pathologic T stage, and positive margins were found in 8 cases (7 with ductal and 1 with lobular carcinoma). We describe our tissue-mapping protocol, and advocate its use as a standardized protocol for processing all partial mastectomy specimens.