A lump in the armpit may be a sign of cancer and should be taken seriously. The most common lump in the armpit is an enlarged lymph node, and the most common site for lymph node metastasis in breast cancer is the ipsilateral armpit lymph node. In some breast cancer patients, the lump can be felt in the armpit when it is still difficult to feel the lump on the breast. Therefore, if a lump is found in the armpit, pay attention to see if there is a lump in the ipsilateral breast. Other cancers, such as lymphoma, may also manifest as lumps in the armpit. Armpit lumps may also be benign tumors, such as subcutaneous lipomas in the armpit area, common breast development malformations in women, and parastomas can also manifest as soft lumps in the armpit area. In conclusion, it is important to be alert to the possibility of cancer in the armpit lumps and also to rule out benign tumors or developmental malformations of the tissue.