The red rash on the buttocks is considered to be related to eczema, jock itch, urticaria, folliculitis, and so on. 1. Eczema: the cause of eczema is not clear, may be related to genetic factors, immune abnormalities, hormone level changes. Eczema will cause the patient local skin erythema, edema, millet size papules or herpes papules, may also be accompanied by flaking, oozing and other symptoms, so it will cause part of the patient buttocks long red rash. 2. Ringworm: ringworm is caused by dermatophyte infection, which is contagious and can be spread through direct contact or indirect contact. Patients are often manifested from the inner to the outer development of the ring or semi-circular erythema, papule skin damage, may also be accompanied by itching, scaling, etc., so some patients with ringworm patients will appear buttocks long red rash phenomenon. 3. urticaria: urticaria is related to allergy, infection, autoimmune and other factors, can cause part of the patient’s buttocks long red rash, may also be accompanied by angioedema, itchiness and other uncomfortable symptoms. 4. Folliculitis: folliculitis is mostly caused by infection with Staphylococcus aureus, skin trauma, excessive sweating and other factors, which can lead to a red rash on the buttocks of some patients, and may be accompanied by itching, pain and other uncomfortable symptoms. It is recommended that the person who has a red rash on the buttocks should consult a doctor for examination, and follow the doctor’s instructions after clarifying the cause of the disease.