Unilateral or asymmetrical axillary adenopathy is very common. The causes are often benign in nature, including reactive changes in the lymph nodes from regional infection (lymphoid hyperplasia) or those due to normal variability. There's a long list of pathologic conditions associated with benign adenopathy: collagen vascular disorders (e.g. rheumatoid arthritis, scleroderma), granulomatous disease (e.g. sarcoidosis, tuberculosis), dermatopathic conditions (e.g. psoriasis, exfoliative dermatitis, infectious rashes), human immunodeficiency virus infection and silicone adenopathy. You'll typically find bilateral axillary node enlargement with lymphoproliferative disorders (e.g. lymphoma, leukemia). In contrast, metastatic breast cancer usually results in unilateral node enlargement in the axilla. Radiographic findings include: poorly defined nodal margins, spiculation, matting, increased density, occasional microcalcification and loss of the normal fatty hilum within breasts. Node metastases can also occur from non-breast primary cancers such as melanoma and lung cancer. Findings of unilateral lymph node enlargement can be seen in up to 1% of women over 40 years old undergoing annual mammography screening. A biopsy is indicated in patients with a history of underlying malignancy or if the lymph node enlarges by more than 100% over baseline. Clinical and mammography follow-up are recommended in those with no history of cancer, minimal nodal enlargement (less than 45 mm) and who have non-palpable and fixed lymph nodes that maintain a benign appearance on radiographic imaging. Intramammary lymph nodes are seen in 5-8% of patients undergoing routine mammography and are variable in size, shape, density and location. Most nodules are round or oval and have well-circumscribed margins. They're often located in the upper outer quadrants and axillary tail regions of the breast. On mammography, they have lucent centres that represent the normal fatty hilum -- its presence is required before a mass located in the upper outer quadrant is assumed to be a lymph node. Turning back now to your clinical case, the diagnosis of "a normal lymph node" on ultrasound combined with the woman's young age would highly suggest that the lump was an incidental finding of a benign lymph node. Breast cancer and other serious conditions can still occur in a younger population, so I would suggest clinical follow-up. Consider ordering unilateral mammography only if the lymph node enlarges or becomes firm or fixed on palpation. If the node remains stable (i.e. doesn't resolve) or the mammography finding is benign, patient reassurance is recommended, as well as annual mammography screening starting at age 40. Finally, to address your last question, breast cancer can occur in men but it's rare (1% of cases). If a young man presents to you with the same findings as above, I would recommend the same clinical management as stated for the young woman. MM [References: Taveras JM, ed. Radiology: Imaging, Diagnosis, Intervention. J.B. Lippincott, Philadelphia, 2003; ch. 93: pp 2-35. Radiology 1997;203:329-31.]