The possible causes of left arm swelling and pain are fracture, muscle injury, neurological disease, etc. You can consult general surgery, orthopedics and neurology. 1. Fracture: For example, closed fracture, the fracture site does not protrude from the body surface, but the fracture occurs inside the arm. The broken end of the fracture rubs against the muscles and nerves, which can cause swelling of the arm, etc. You should consult an orthopedic surgeon. 2. Muscle injury: repeated lifting of heavy objects, playing tennis, badminton, etc. can cause tenosynovitis and other induced aseptic inflammation, you can consult the general surgery, orthopedics. Generally, rest, hot compresses and oral anti-inflammatory drugs can help reduce symptoms. Most of them are self-healing diseases and can be relieved by resting for 1 to 3 weeks. 3. Nerve diseases: There are median nerve, radial nerve and ulnar nerve in the arm. The nerves are connected upward to the nerve root. If the nerve root is compressed due to cervical spondylosis, etc., pain and swelling in the innervated area can be produced, and you can register with neurology and orthopedics. Treatment of the primary cause is usually required, such as pulling and surgery for cervical spondylosis. If there is left arm swelling and pain, you need to seek medical treatment as soon as possible to avoid serious complications.