Possible causes of left posterior upper back pain include rotator cuff injury, frozen shoulder, shoulder fasciitis, cervical spondylosis, cervical disc herniation, angina pectoris, myocardial infarction and other diseases. 1. Shoulder-related diseases: rotator cuff injury, shoulder fasciitis and frozen shoulder can cause pain in the left shoulder and upper back. 2. Cervical spine diseases: including cervical spondylosis and cervical disc herniation, which stimulate the cervical nerves and lead to pain and numbness in the innervated left shoulder and back. 3. Heart disease: such as angina pectoris, myocardial infarction and other attacks, also often have radiating pain in the back of the left shoulder, accompanied by chest pain, chest tightness, shortness of breath and other discomforts, it is recommended to immediately seek medical attention to avoid life-threatening. In addition, a small number of gastric ulcer, pancreatitis, pleurisy, intercostal neuralgia, etc. will also appear in the left back pain, it is recommended to consult a doctor in a timely manner, to identify the cause of the disease, standardized treatment under the guidance of the doctor, in order to avoid progression or deterioration of the condition.