Red plaques on thighs may be caused by erythema nodosum, purpura allergic, psoriasis and other diseases. 1. Erythema nodosum: the etiology is complex, generally believed to be related to infections, especially streptococcal infections and drug reactions. Erythema nodosum is a red subcutaneous nodule occurring on both lower legs, which may subside by itself or be a concomitant symptom of other diseases. Typical damage is bright red, painful subcutaneous nodules and plaques on the front of both shins, which may have pressure pain. 2. Subacute nodular wandering sebaceous meningitis: nodular erythema-like rash appears in the calf, usually early in the course of the disease can occur unilaterally, painless, centrifugal enlargement, the edge of the bright red, the center of the whitening, and can be gradually flattened to form plaques, the size of 10-20 cm, the duration of two months to two years, the performance of the hyperpigmentation, also known as wandering erythema nodosum. 3. Psoriasis: the cause of psoriasis is not yet clear, may be genetic, pathogen infection, stress, obesity and other factors. Patients will have red plaques on the thighs, which can be covered with silver-white scales, accompanied by itching, burning pain and other symptoms. There are many reasons (such as urticaria, hard erythema) that lead to the appearance of red plaques on the thighs of patients, patients need to go to the hospital, identify the cause of the disease in a timely manner, and treatment under the guidance of the doctor.