When left upper chest pain occurs, it is necessary to be alert to cancers such as breast cancer and lung cancer; it may also be non-cancer diseases such as heart disease. 1. Breast cancer: it is a common malignant tumor in women, which occurs in the left upper thoracic quadrant of the breast, and may present specific symptoms such as local lump, pain and orange peel-like changes; it may also be accompanied by systemic manifestations such as fever, fatigue, and unexplained emaciation. 2. Lung cancer: it is a common malignant tumor of the respiratory system. When the lesion appears in the left side of the lung, it may present clinical manifestations such as left upper chest pain, coughing and sputum, hemoptysis, etc., and the same systemic manifestations as breast cancer. 3. Heart disease: it may also be non-cancer factors leading to left upper chest pain, such as coronary heart disease, myocardial infarction and so on. It is recommended to consult a doctor in time after the discomfort occurs, so as to clarify the cause of the disease and treat the cause as early as possible.