Flat warts need to be determined based on clinical features, the presence of isomorphic reactions and histopathologic examination. 1. Clinical features: flat warts are skin-colored or light brown flat papules that usually occur in children and adolescents, mostly on the face, forearms and back of the hands. Typical manifestations are rice to soybean-sized flat papules, round or oval, hard texture, scattered distribution in the above areas. 2. Homomorphic reaction: usually after scratching or injury, the lesions will appear in the local bead-like arrangement of small papules, and then gradually develop into a flat papule pattern, known as homomorphic reaction or koebner phenomenon. 3. Pathology: Flat warts are characterized by vacuolization of the granular layer of the skin, the upper part of the stratum spinosum, and viral particles observed under the electron microscope. It is recommended that patients with flat warts or suspected flat warts go to the hospital as soon as possible for individualized diagnosis and treatment by the doctor to avoid delays in treatment.