Abstract: Objective: To study the clinicopathologic and immunohistochemical characteristics of primary invasive mi -cropapillary carcinoma (IMPC) arising in the axillary accessory mammary gland. Methods:The patients underwent wide lo-cal excision then pathologists examined the whole specimen in a series of sections. Immunohistochemical staining of ER, PR, cerbB- 2, P 53, epi thel ial membrane antigen (EMA), E-cadherin, α-Catenin and β-Catenin was performed. Results: The tumor was composed of IMPC and ductal carcinoma in situ, accounting for 95% and 5%, respectively. Normally positioned breast tissue was seen beside the axillary tumor. The neoplasm was diagnosed as primary IMPC arising in the axillary ac-cessory mammary tissue. Lymphatic vessels with invasion were seen within the tumor and metastases were detected in 35 of the 43 axillary lymph nodes. Morphologic appearance of the carcinoma in the lymphatic vessels and positive lymph nodes was the same as in the primary carcinoma. Immunohistochemistry showed that tumor cell clusters were positive for ER and PR and negative for P 53and cerbB- 2. A peculiar immunoreactivity for EMA limited to the cytoplasmic membrane oriented toward the stroma and an absence of immunoreactivi ty for E-cadherin, α-Catenin and β-Catenin in the same side of the cytoplasmic membrane indicated inversion of cell polarization and a disturbance in the cell adhesion molecules. Conclusion: The clinicopathologic and immunohistochemical characteristics of the IMPC arising in axillary accessory mammary glands are identical with the IMPC of the breast, with high potential of invasion and metastasis.