Breast development on one side of an 8.5 year old girl can be caused by true precocious puberty, pituitary tumor or simple breast development, etc. It can be treated with general treatment, medication such as treprostinil, and surgery if necessary. 1. True precocious puberty: it is a kind of true precocious puberty caused by the activation of hypothalamus-pituitary-gonadal axis, and the order of development is usually normal, starting from the mammary glands. It is necessary to develop good living habits, control body weight and exercise, and avoid eating exogenous estrogen. Drugs such as treprostinil are mainly used to control true precocious puberty. 2. Pituitary tumor: generally due to benign tumors of the pituitary gland, a kind of precocious puberty, usually need to be combined with surgical resection, radiotherapy and other treatments. 3. Simple breast development: generally not accompanied by the initiation of the hypothalamus – pituitary – gonadal axis, bone age, hormone five tests are also no abnormality, only manifested as breast development, this time can be observed, regular review can be. There are other reasons for breast development in 8 and a half year old girls, it is recommended to consult a doctor in a timely manner, improve the examination, identify the cause of the disease, treat the cause of the disease, and follow the doctor’s instructions for the use of the above medication.