Hidden pain in the rib cage under the left armpit is generally considered to be chest soft tissue contusion, costochondritis, and chest tumor disease. 1. Chest soft tissue contusion: Hidden pain in left armpit rib cage may be caused by chest soft tissue contusion. There is usually a history of trauma, which will be more obvious when exertion, activity, turning over, deep breathing or loud talking. It is recommended to take some non-steroidal anti-inflammatory and analgesic drugs for treatment. At the same time, avoid eating spicy food, drink more water, rest, and ensure enough sleep. 2. Costochondritis: it is the inflammatory reaction of the cartilage at the junction of sternum and ribs, the pain is mostly in the cartilage near the sternum, the cause is usually unknown, and the pain may be aggravated when moving. Local physical therapy and acupuncture can be considered to relieve the symptoms. 3. Chest tumor: usually it is malignant tumor of the left lung, and the cancer cells will invade the ribs, causing hidden pain in the left axillary ribs. It is recommended to improve lung CT to determine the location of the tumor and then perform surgery under the guidance of doctor. Left axillary rib pain may also be caused by other reasons, so it is recommended to consult the doctor in time to clarify the cause of the disease and then carry out treatment in time.