The significance of micrometastasis (MiM) versus macrometastasis (MaM) in breast carcinoma (BC) impacts pathology staging and management. Current guidelines recommend that axillary lymph nodes (LNs) should be considered negative if the deposits are less than 2 mm. Two different mechanisms can account for this occurrence: (1) iatrogenic displacement of cancer cells from the primary site, secondary to core-needle biopsy, to a draining LN, or (2) true metastasis. Ideally, pathology diagnosis and staging should reflect the true status of the LN involved by BC in the individual patient. To address this issue, we performed comparative mutational profiling of primary BC and corresponding MiM and MaM LN deposits. A total 14 cases of breast ductal carcinoma with nodal MiM and 9 control cases consisting of breast ductal carcinoma with nodal MaM were retrieved from the hospital computer system following institutional review board approval. All cases were confirmed by histology and immunohistochemistry. Directed microdissection used unstained formalin-fixed, paraffin-embedded standard tissue sections, following correlation with the hematoxylin-eosin slides. Microdissection targets consisted of 1 nonneoplastic site and 2 to 3 sites of primary BC and LN metastatic deposits. Each microdissected sample was quantitatively genotyped, and the time course of mutation acquisition was determined using a broad panel of 17 mutational markers including loss of heterozygosity for 1p, 3p, 5q, 9p, 10q, 17p,17q, 21q, and 22q. Mutational heterogeneity was controlled in each case by the use of replicate microdissection targets. All control cases of MaM disease showed the same number or greater cumulative mutations in LN deposits than the primary site, and this was used to define 2 states: metastasis versus passive spread (LN deposit showed fewer cumulative mutations than primary tumor). Of the 14 cases with MiM, 9 (64.3%) of 14 showed fewer mutations in the nodal metastases than the primary breast tumor, supporting passive displacement rather than true metastasis. Five (35.7%) of 14 cases showed increased number of mutations, supporting true metastases. Replicate analysis demonstrated that molecular heterogeneity was not an important factor affecting mutational profiling or conclusions. None of the MiM cases demonstrated histologic findings associated with iatrogenic passive displacement of malignant cells to axillary LNs. Our results support the need for individualizing pathologic staging of BC with respect to the biological nature of MiM disease. We believe molecular analysis is a useful technique to better evaluate the significance of MiM in axillary LNs. Correlative studies with outcome analysis would be helpful to determine the significance of our findings.