Vague pain in the left chest is usually produced by stimulation of the left anterior chest wall and thoracic organs and tissues nociceptive nerve endings stimulated by inflammatory factors. It is common in costochondritis, mediastinal disease, heart disease, pleural lung disease, upper gastrointestinal tract disease, etc. 1. Costal chondritis: chronic non-specific inflammation, with vague pain at the junction of costal cartilage and sternum in the left chest. 2. Mediastinal diseases: anterior mediastinal tumors such as thymoma invading the post-thoracic tissues may also cause vague pain in the left chest. 3. Heart disease: manifested as vague pain in the precordial region, mostly seen in angina pectoris, coronary atherosclerotic heart disease, myocarditis caused by viral infection, pericarditis and other conditions. 4. Left pleurisy, lung infection, limited pleural tumor, lung cancer invading the back of the sternum, etc. also cause vague pain in the left chest. 5. Upper gastrointestinal diseases: such as early esophageal cancer, esophageal hiatal hernia, reflux esophagitis, gastritis, etc. can also cause vague pain behind the sternum. It is recommended to go to the hospital as soon as possible for further diagnosis and treatment to avoid delaying the condition.