Breast Cancer: This is often the first consideration in cases of axillary lymphadenopathy, especially in women, due to the direct lymphatic drainage from the breast to the axillary lymph nodes. The presence of a breast mass or other symptoms like nipple discharge can support this diagnosis.

Infectious Causes (e.g., Cat Scratch Disease, Tuberculosis): These can cause axillary lymphadenopathy, especially if there's a history of exposure to cats or travel to areas endemic for tuberculosis.
Lymphoma: Both Hodgkin's and non-Hodgkin's lymphoma can present with lymphadenopathy, including in the axilla. Systemic symptoms like fever, night sweats, and weight loss may accompany lymphoma.
Reactive Lymphadenopathy: This can occur in response to infections (viral or bacterial), vaccinations, or other inflammatory conditions. It's often self-limiting and resolves once the underlying cause is addressed.

Metastatic Malignancy: Besides breast cancer, other malignancies (e.g., lung, melanoma, gastrointestinal cancers) can metastasize to axillary lymph nodes. A thorough history and examination are crucial to identify potential primary sites.
Sarcoma: Although rare, sarcomas (e.g., soft tissue sarcomas) can present with axillary lymphadenopathy. Early detection is critical for treatment outcomes.
Lymphoma (again, due to its potential severity and the importance of early diagnosis): While listed under other likely diagnoses, lymphoma's potential severity and the importance of early diagnosis warrant its inclusion here as well.

Kikuchi-Fujimoto Disease: A rare, self-limiting condition that predominantly affects young women, characterized by lymphadenopathy, often in the neck but can be axillary, and systemic symptoms.
Kimura Disease: A chronic inflammatory disorder that can cause lymphadenopathy and eosinophilia, more common in Asian males.
Castleman Disease: A rare disorder that can cause lymphadenopathy, among other symptoms, due to overgrowth of cells in the lymph nodes.