Red patches on the roots of the thighs may be related to contact dermatitis, ringworm, eczema and other diseases. 1. Contact dermatitis: the cause of the disease is mainly related to the allergic reaction caused by skin contact with exogenous substances, and common exogenous substances include soap, clothes and so on. Patients will appear local erythema, papules, edema and other symptoms. 2. ringworm: the cause of the disease is mainly related to canine microsporidium, red trichophyton and other pathogenic fungal infections. Patients will appear localized ring-shaped erythema, can be seen in the groin, buttocks and other parts. 3. Eczema: the cause of the disease may be related to immune abnormalities, genetic and other factors, the patient will appear localized erythema, papules, papules and other symptoms, lesions can be seen on the whole body. If there are red patches on the roots of the thighs, it is recommended to go to a regular hospital for examination to clarify the cause.