Underarm rib pain is caused by inflammation, infection, tumor and other pains in the tissues of the nearby organs under the armpit. It is commonly seen in costochondritis at the rib arch, intercostal neuritis or herpes zoster, pleurisy, swollen lymph nodes in the armpit, carcinoma of the lung tip, and digestive system diseases such as gastric ulcer. 1. Costal chondritis at the costal arch: the costal cartilage at the costal arch may appear swollen and elevated and localized pain. 2. Intercostal neuritis or herpes zoster may reach the axilla along the intercostal space and cause subaxillary ribs. 3. Pleurisy: Pleurisy with friction between the wall and dirty pleura may cause pain in the axillary ribs in the anterior and mid-axillary lines. 4. Enlarged lymph nodes in axilla: benign or malignant enlarged lymph nodes caused by various kinds of mastitis, breast abscess, breast cancer, etc. can cause axillary rib pain. 5. Lung apical cancer: compression of brachial plexus nerve by lung apical cancer can cause pain in upper limbs and ribs under axilla. 6. Digestive system diseases: gastric ulcer, acute pancreatitis and other diseases may involve pain to axillary rib pain. Patients with axillary rib pain are recommended to go to hospital for further examination and diagnosis in time.