Enlargement of the armpit-back junction is usually caused by folliculitis, sebaceous cysts, enlarged lymph nodes and inflammation of the costal cartilage. 1. Folliculitis: There are abundant hair follicles under the armpits. If the hair follicles in the armpit-back junction become inflamed due to bacterial infection, the blood vessels distributed in the area may be dilated, resulting in localized swelling of the skin. 2. Sebaceous cysts: When the sebaceous ducts in the skin of the armpits and the back are blocked and sebum accumulates to form sebaceous cysts, you can feel the enlarged sarcoma-like objects under the skin. 3. Enlarged lymph nodes: If the lymph nodes distributed in the connection between armpits and back are inflamed, resulting in proliferation of lymphocytes or poor return of lymphatic fluid, it can cause enlarged lymph nodes, which can be manifested as enlarged armpits and back connection. 4. Inflammation of rib cartilage: If the rib cartilage is inflamed due to chronic strain, local trauma and infection, the infiltration of inflammatory cells leads to edema of the rib cartilage, which may cause enlargement of the connection between the armpit and the back. Patients with enlarged armpit-back junction may also have other causes, such as lipoma and fibroma. The patient should seek prompt medical attention, make a definitive diagnosis and follow the doctor’s instructions.