Pain under the left armpit may be caused by trauma, costochondritis, lymphadenitis and other diseases. 1. Trauma: usually the patient’s armpit is hit or squeezed by external force, resulting in localized soft tissue contusion, which will cause the symptoms of left axillary pain. 2. Costal chondritis: the etiology of non-specific costal chondritis is still unclear, and it is considered to be related to viral infection, chronic strain of thoracic and costal joint ligaments, immunity and other factors. Infectious costochondritis is due to surgery, trauma and other conditions. Patients often have anterior chest pain, which can also radiate to the back or abdomen, and some patients may also have pain in the left armpit. 3. Lymphadenitis: usually due to hemolytic streptococcus, staphylococcus aureus infection, and related to cold, upper respiratory tract infection and other factors. Patients often have swollen and painful lymph nodes, and if patients have axillary lymphadenitis, they may have pain under the left armpit. It is recommended that patients with symptoms of pain in the left armpit actively identify the cause of the disease and follow the doctor’s instructions for treatment to avoid delaying the condition.