Hard lumps in a girl’s chest may be related to breast development, precocious puberty, sebaceous cysts, boils and other causes. 1. Breast development: During puberty, breast tissue will appear on the girl’s chest and feel like hard lumps. 2. Precocious puberty: precocious puberty girls may develop hard lumps in their breasts before the age of 8. 3. Sebaceous cysts: If the opening of the sebaceous gland in the chest is blocked, resulting in the secretion not being able to be discharged and forming a cyst, it manifests as a hard lump. 4. Boils: when a folliculitis in the chest is infected by bacteria, it can cause local inflammatory reaction, lumps, pain and other symptoms. Parents are advised to take their children to the hospital in time to clarify the cause, and if treatment is needed, standardize the treatment under the guidance of the doctor.