A red rash on the inner thighs is not necessarily AIDS, but should also consider the possibility of other diseases such as eczema, atopic dermatitis and syphilis. 1. AIDS: In the acute infection stage, patients may develop a rash, which can be manifested as a red rash on the inner thighs. Patients may also experience other symptoms at the same time such as headache, fever, vomiting and diarrhea. 2. Eczema: It can lead to lesions with red papules on the affected area, accompanied by other symptoms such as itching, pain, swelling and heat. 3. Atopic dermatitis: lesions are more diverse, some patients may have localized red rash, itching, stinging and other symptoms. 4. Syphilis: Syphilis stage II can appear syphilis rash, can also be manifested as a red rash on the inner thigh. Rash on the inner thighs may also be other diseases, it is recommended to consult a doctor to confirm the diagnosis and treatment as soon as possible.