Tumor size is a significant prognostic indicator for invasive mammary carcinoma. By current standards, this is routinely reported during pathologic evaluation of the definitive excision, but no recommendations exist for reporting tumor size in needle biopsy material. The purpose of this study is to evaluate the relationship between tumor size on breast needle biopsy specimens and that on subsequent definitive excision specimens and to evaluate the impact of the former, if any, in determining the final pathologic tumor stage. This was achieved by an evaluation of 222 consecutive cases of invasive mammary carcinoma for which both the diagnostic biopsy and definitive excision were available for review. Of 200 cases without a history of neoadjuvant therapy, there were 161 (80.5%) cases in which the tumor size on biopsy was smaller, 15 (7.5%) cases in which the sizes were equal, and 24 (12%) in which the size on biopsy was greater, including 6 (3%) cases with no residual tumor on excision. The average size change (excision size minus biopsy size) increased with increasing tumor stage, with these being significantly lower in pT1a compared with pT1b tumors (-0.14 vs. 0.17 mm; P=0.0002), pT1a/b compared with pT1c tumors (0.12 vs. 0.53 mm; P<0.0001), and pT1 compared with pT2/3 tumors (0.32 vs. 2.2 mm; P<0.001). Of the 24 cases in which tumor size on biopsy was greater than that on excision, there were 15 (7.5% of cohort) in which the tumor size on biopsy was the sole determinant of a higher final pathologic T stage. A larger tumor size on biopsy compared with that on excision was significantly associated with a lower final pathologic T stage (P<0.001) but not with patient age, histologic type, histologic grade, mitotic score, or the presence/absence of ductal carcinoma in situ. Evaluation of the remaining 22 cases also showed that there was a clear association between a history of neoadjuvant therapy and the finding of a larger size on biopsy compared with that on excision (P<0.0001). These findings indicate that tumor size on breast needle biopsy is not infrequently larger than that on excision and can also dictate the final pathologic T stage. Accordingly, it is recommended that the greatest extent of invasive carcinoma is reported in all needle biopsy specimens.