Pain in the left rear shoulder can be caused by the following: 1. It can be caused by frozen shoulder on the left side. Usually there is pain in the left shoulder, which is also accompanied by widespread pain elsewhere in the shoulder joint, such as anteriorly or laterally. Symptoms are also accompanied by the left side of the shoulder joint activity function is limited, or shoulder joint activities when the pain will be aggravated. 2, may be the left side of the shoulder back of the myofascial inflammation, usually is the local muscle strain or excessive exertion and the formation of chronic aseptic inflammation, usually through the external application of ointment or oral non-steroidal anti-inflammatory drugs can be cured. 3, must pay attention to whether there is a cardiac problems, acute Myocardial infarction or angina pectoris may cause pain in the left shoulder, but it is usually accompanied by pain in the precordial area or chest, chest tightness, palpitations and other symptoms.