Darkening of the thigh triangle can be divided into physiological and pathological. Physiological causes are related to repeated skin friction, while pathological causes are related to acanthosis nigricans and endocrine disorders. If physiological reasons are excluded, it is recommended that patients go to the hospital in time. 1. Physiological: Most of the blackening of the thigh triangle is related to the repeated friction of clothes on the thigh triangle, which causes excessive melanin synthesis. It may also be caused by the patient’s prolonged sitting, resulting in melanin precipitation. 2. Pathologic: (1) Acanthosis nigricans: patients suffer from acanthosis nigricans, which causes excessive skin pigmentation, leading to the symptoms of darkening of the thigh triangle. (2) Endocrine disorders: patients have unbalanced hormone levels in the body, resulting in excessive melanin secretion, causing darkening of the thigh triangle. Patients may also have symptoms such as macular, darkening of the face, short temper, obesity and so on. If physiological reasons are excluded, it is recommended that patients go to the hospital in a timely manner to clarify the cause of the disease.