A lump in the armpit is caused by the following reasons: 1. a body surface swelling, such as lipoma, sebaceous cyst, fibroma, etc., need to go to the hospital to improve the relevant examination, such as blood routine, ultrasound, etc., if it is a lipoma does not need to be removed, if it is a sebaceous cyst, need to improve the relevant examination as soon as possible to remove, in order to prevent the emergence of rupture and infection; 2. it may be the formation of parametrial, common in pregnancy and postpartum If it does not subside or even continues to grow, it needs to be removed surgically; 3. It may be caused by enlarged lymph nodes, and there are many causes, such as bacterial infection, viral infection, or even lymph node metastasis of malignant tumors.