The appearance of red marks on the back for no apparent reason is most likely related to contact dermatitis, lichen planus, eczema, acute urticaria, hemangioma, acne and other causes. 1. Contact dermatitis: If your back comes into contact with allergens such as metal, it may cause red marks to appear on the area in the same shape as the contact material. 2. Lichen planus: When lichen planus is present, a well-defined rash of light brown or reddish spots may appear. 3. Eczema: Eczema patients usually present with pimples and papules on top of erythematous skin patches, which may appear as red marks at an early stage. 4. Acute urticaria: due to insect bites, infections and other factors causing reactive expansion of the skin mucosa and small blood vessels, red marks can appear. 5. Hemangioma: Hemangiomas are bright red, after which the color gradually deepens and changes from bright red to dark purple, which is in the form of a blossom before subsiding. 6. Acne: most often appearing on the face, shoulders or back and other parts of the face, after the lesions can appear red spots. It is recommended that people with red marks on the back for no reason go to a regular hospital in time to clarify the cause of the disease and standardize the treatment under the guidance of the doctor to avoid delaying the condition.