The darkening of the skin on the inner thighs may be caused by physiological factors such as friction, or it may be considered to be caused by diseases such as ringworm and acanthosis nigricans. 1. Physiological factors: if the inner thighs long-term friction, may lead to local skin pigmentation, appearing thigh skin black. Especially obese people, prone to such situations. It is recommended that obese people, according to their own situation, take a reasonable way to lose weight, which can alleviate such a situation. 2. Ringworm: a disease caused by the infection of dermatophytes, which can lead to red, reddish-brown, light brown plaques or papules on the inner thighs, and can also cause darkening of the inner thighs. 3. Acanthosis nigricans: mostly related to heredity, obesity, tumor, etc. Clinically, it mainly manifests itself as darkening and thickening of the skin folds, which can also cause darkening of the skin on the inner thighs. There are many reasons for post-inflammatory hyperpigmentation, and patients can go to regular hospitals to find out the cause.