There are many reasons for the blackening of the inner thighs, such as excessive friction, pseudoacanthosis nigricans, excessive post-inflammatory hyperpigmentation and so on. 1. Excessive friction: If the patient’s body is relatively obese, pants are too tight, etc., it may lead to long time friction and stimulation of the inner thighs, which will lead to the phenomenon of blackening of the inner thighs. 2. Pseudoacanthosis nigricans: Pseudoacanthosis nigricans is common in obese people, which occurs at the root of thighs, inner thighs, armpits, neck and other locations, and can lead to the phenomenon of blackening of the inner thighs. 3. Excessive post-inflammatory hyperpigmentation: darkening of the inner thighs may also be excessive post-inflammatory hyperpigmentation, if there is chronic eczema, dermatitis, fungal infection in the inner thighs, it will cause post-inflammatory hyperpigmentation, which will lead to darkening of the inner thighs. There are other reasons for the darkening of the inner thighs, and it is recommended to go to the hospital in time to improve the examination after the symptoms appear, and take standardized treatment according to the doctor’s instructions, and not to ignore the symptoms.