Nine-year-old girls with hairy legs may be caused by genetic factors, disease factors and so on. 1. Genetic factors: may be due to chromosomal aberrations or intergenerational inheritance, there will be congenital leg hair is more, the growth of hair in other parts of the body tends to be more exuberant. 2. Disease factors: often caused by various diseases leading to excessive secretion of androgens in the body, such as adrenocortical hyperplasia, hypothalamic tumors or pituitary tumors, and so on, may cause this phenomenon. 9-year-old girl leg hairy may also have other reasons, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.