Symmetrical erythema on both inner thighs has more causes, which may be tinea cruris, eczema, familial chronic benign aspergillosis, stage II syphilis, etc.: 1. Tinea cruris: Tinea cruris will appear as symmetrical erythema with clear boundaries in the groin of the inner thigh, and may be accompanied by scaling and itching. Because the affected area is poorly ventilated, humid, and prone to friction, it often makes the inflammation of the lesions obvious and the itching significant; 2. eczema: eczema is mostly formed by allergic factors or local irritating factors, and can be induced when the skin of the legs is stimulated such as dryness, heat, and hot water scalding, etc. It will appear symmetrical erythema on the inner thighs, and some people also have obvious symptoms such as itching, papules, and papules; 3. familial chronic benign Pemphigus: Familial chronic benign pemphigus is a rare autosomal dominant disease with lesions occurring in the neck, axillae and groin. The symmetrically distributed erythematous base may have flaccid blisters, with self-induced itching and burning and a fishy odor. A few of them may be accompanied by oral and vulvar mucosal damage. In summer, the lesions are aggravated by sweating, and the inter-rubbing area is often painful due to maceration or chafing; 4. Phase II syphilis: syphilis is a chronic sexually transmitted disease, mainly through sexual contact and blood transmission. The syphilis rash of second-stage syphilis can also be manifested as symmetrical erythema on the inner thighs, which can be diagnosed and ruled out by serological testing at the hospital.