Slight pain in the left breast can be seen in breast hyperplasia, heart disease, respiratory disease, digestive disease and so on. 1. Mammary hyperplasia: Mammary hyperplasia is a common benign breast lesion in women, which manifests itself as swelling and pain in the breasts, and the symptoms worsen before the onset of menstruation. Therefore, women with breast enlargement may experience pain in the left breast. 2. Heart disease: most of the heart is located in the left side of the chest cavity, such as myocarditis, pericarditis, coronary artery disease and other lesions of the heart, may appear in the left chest pain, but also can be accompanied by chest tightness, panic, shortness of breath and other discomforts. 3. Respiratory system lesions: such as pneumonia, tuberculosis, lung abscess and even lung cancer, etc., such as the adhesion of the lesion to the pleura can cause stabbing chest pain, dull pain and other symptoms, often accompanied by coughing, coughing up sputum, dyspnea and so on. 4. Digestive tract lesions: such as reflux esophagitis, etc., because the esophagus is located in the back of the sternum, so clinically it can be manifested as chest pain, accompanied by burning sensation and so on. There are many other causes of mild pain in the left chest, it is recommended to go to the hospital for further examination to clarify the cause of the disease and timely treatment, so as not to delay the condition.