Objective: The clinical meaning of intramammary lymph nodes (iMLNs) is uncertain. We wanted to describe the clinical characteristics and understand the implications of iMLNs detected by sentinel LN biopsy (SLNB). Methods: We reviewed the clinical, radiological and pathological records of women diagnosed with invasive carcinoma of the breast at the Samsung Medical Center between January 2001 and January 2011. A total of 69 patients were identified, and SLNB was performed in 31 patients. Results: Of the 69 patients included in the study, 22 (31.9%) had metastases in iMLNs. The presence of lymphovascular invasion and the number of axillary LN metastases were associated with iMLN metastasis. Of the 31 patients who underwent SLNB, there were no cases with axillary LN metastasis when axillary SLNB was negative, even in cases of iMLN-positive patients. Of the 10 patients in whom intramammary SLNs (iMSLNs) were detected during SLNB, 8 patients without iMLN metastases were also negative for axillary LNs. Conclusion: Patients with metastatic iMLNs had more aggressive cancers with lymphovascular invasion and increased axillary LN metastases. When iMSLN was detected by SLNB, performing of axillary dissection could be determined by the status of the iMSLN itself.