The darkening of the inner thighs of girls may be caused by physiological factors such as friction, or it may be caused by diseases such as acanthosis nigricans and post-inflammatory hyperpigmentation. 1. Physiological factors: If clothes are usually worn too tightly, there will be friction when walking around, and in the long run, girls may have black inner thighs. 2. Acanthosis nigricans: it is usually characterized by dryness, roughness and deepening of the color of the skin, which can be seen on the limbs, hands and feet. The cause of the disease is not yet clear, and it may be caused by heredity, low immunity, drugs and other factors. 3. Post-inflammatory hyperpigmentation: its causes are more varied and may be related to diseases, trauma and environmental factors. Typical symptoms include the occurrence of inflammation in the area will appear not prominent skin surface rash or patches, light brown, black, dark brown and so on. There are more reasons that lead to the darkening of the inner thighs of girls, and after eliminating physiological factors, it is recommended that the patient consult a doctor for examination and symptomatic treatment after clarifying the cause.