Children with symmetrical erythema behind the knee are considered to be allergic purpura, contact dermatitis, eczema, etc., which can be treated with local and oral medication. 1. Anaphylactic purpura: this is an allergic inflammation of small blood vessels throughout the body. It is considered to be exposure to allergens, leading to an increase in capillary fragility and permeability, and should be kept away from allergens. Treatment can be injected with anti-allergic drugs, such as vitamin C injection, calcium gluconate injection and so on. If the joint pain also need to use non-steroidal anti-inflammatory drugs, such as ibuprofen, acetaminophen. 2. Contact dermatitis: If contact with allergic substances, contact dermatitis will occur, mildly manifested as erythema, mild edema, can be applied locally mometasone furoate cream, dexamethasone cream, itching is serious, you can take oral loratadine, ibuprofen. 3. eczema: eczema is a chronic inflammation of the skin, there may be papules or small blisters, basal redness, symmetrical distribution, if there is no blisters vesicles oozing, you can apply glycerine lotion, dexamethasone cream, if there is a large amount of oozing, you can use boric acid solution, isacridine solution for wet compresses, there are vesicles, but not much exudation when you can use zinc oxide oil. Children have symmetrical erythema behind the knee, should go to the hospital, by the doctor to clarify the cause of symptomatic treatment, the above drugs should be used in accordance with the doctor’s instructions.