Buttock crevice on both sides of the buttocks, that is, on both sides of the posterior midline, redness of the skin accompanied by pain, may be local trauma infection, soft tissue blunt contusion, contact dermatitis and other causes. 1. Local trauma infection: local skin infection due to skin abrasion, etc., can cause injury, in addition to redness, swelling, ulceration and pus, and other manifestations, the serious may lead to generalized fever and other symptoms. 2. Soft tissue blunt contusion: soft tissue injury to the buttocks due to sudden fall, collision and other reasons, may cause local skin redness, swelling or bruising, accompanied by a clear location of pain and pressure. 3. Contact dermatitis: due to long-term sedentary local moisture, contact with dust and other foreign objects, may lead to skin contact dermatitis, manifested as local redness, swelling, pain, and may also be accompanied by itching and other symptoms. Butt crack on both sides of the red, swelling, pain and other symptoms may also be other reasons, it is recommended to go to the hospital in a timely manner.