Pain in the left upper arm depends on whether the left upper arm is muscular and soft tissue, or bones and joints. Each of these areas may have certain signs and symptoms of disease, and the patient should be given the appropriate physical examination based on these signs and symptoms. The first step is to take a medical history, then do a physical examination and then determine which tests to do. If there is soft tissue swelling and pain in a fixed bone, a CT or MRI must be done. If there is joint pain, make sure to check the movement of the joint and also do a CT or MRI. If a tumor is suspected, a tumor marker test is also needed. If a specific mass is found, a needle prick should be done, a pathological examination with a puncture histology biopsy, or peripheral circulating blood can be taken for a liquid biopsy of cancer cells or protein fragments. In conclusion, for upper arm pain, it is important to first distinguish whether the soft tissues, bones, and nerves are of the joint or a particular part of the limb, and then physical examination. The specific examination includes pathological examination, and finally the disease is diagnosed and then treated accordingly. If it is not a malignant disease, but only a functional disorder, or an old age disease, symptomatic treatment can be given according to old age disease.