The red rash on the inside of the bend of the arm may be caused by contact dermatitis, prickly heat, urticaria and other diseases. 1. Contact dermatitis: contact dermatitis occurs when the skin comes into contact with exogenous allergens, triggering an allergic reaction, common exogenous substances are metals, dyes, clothing, etc., lesions are mostly confined to the contact area, manifested as erythema, pimples, oedema, blisters, etc., so the red rash on the inner side of the arm bend may be caused by contact dermatitis. 2. prickly heat: this disease is mainly due to sweat duct obstruction affecting the sweat discharge and caused by the hot and stuffy environment, sweat duct underdevelopment, strenuous exercise and other factors. The red rash on the inner side of the arm bends is mostly considered to be prickly heat, which can occur on the trunk, elbow socket and other places. 3. Urticaria: The occurrence of this disease is related to food allergies, pathogenic microbial infections, sun exposure, mental stress, friction and other factors, but also secondary to rheumatic fever, systemic lupus erythematosus and other diseases. The main symptom is the appearance of red or pale white skin masses, accompanied by itching and edema, which can be manifested as a red rash on the inner side of the bend of the arm. It is recommended that patients with red rash on the inside of the crook of the arm go to the hospital to clarify the cause of the disease.