A 7-year-old girl with a hard lump in one breast may be caused by precocious puberty and breast tumors, and she needs to consult a doctor in time to find out the cause. 1. Precocious puberty: Under normal circumstances, the pubertal development of girls is at the age of 10-14 years. If a girl has a hard lump in one breast at the age of 7, it is likely to be a sign of precocious puberty. It is necessary to consult a doctor in time to determine the child’s condition and to diagnose and treat it. 2. Breast tumor: a 7-year-old girl with a hard lump in one breast may also be caused by a breast tumor, in which case the lump is hard and the mobility will be poor, and a biopsy is needed to make a clear diagnosis. If it is caused by a tumor, it needs to be treated with surgery under the guidance of a doctor. 7-year-old girl with a hard lump in one breast should also consider trauma, chest wall diseases and other reasons, when the situation, should promptly consult a doctor for examination, clear cause and follow the doctor’s instructions for treatment.