Enlarged lymph nodes under the armpit may be caused by lymphoma, breast hyperplasia, breast nodule, breast cancer, mastitis, etc. The specific cause of the disease needs to be confirmed after examination in regular hospitals. 1. Lymphoma: related to heredity, radioactive irradiation, estrogen and other factors, when breast cancer invades peripheral lymphatic ducts, it can be transferred to the local lymphatic drainage area, and the initial stage of the patient may have the symptom of enlarged lymph nodes under the armpit. 2. Breast hyperplasia: due to the endocrine function disorders of patients, it will cause breast hyperplasia, which may lead to lymph node enlargement besides manifesting as breast pain, nodules and lumps. 3. Breast nodules: secondary inflammation in patients with breast nodules may lead to inflammation of axillary lymph nodes and symptoms of enlarged axillary lymph nodes. Enlarged lymph nodes in the axilla of breast nodules may also be caused by breast cancer and inflammation of the breast. If such symptoms occur, it is recommended to consult a regular hospital in time to clarify the cause of the disease and then carry out targeted treatment or therapy.