Horizontal lines on the fingernail cover of the hand may be caused by diseases such as lichen planus, nail fungus disease, psoriasis and so on. 1. Lichen planus: research has shown that the causes of lichen planus include heredity, abnormal immune response, viral infection, mental stress, long-term exposure to chemicals and other factors. Patients will have hyperkeratosis, thickening and longitudinal fissures under the nails, which can be manifested as transverse lines on the fingernail caps. 2. Nail fungal disease: this disease is usually caused by fungi such as Trichophyton rubrum, Trichophyton mustelii and Candida. The nail plate of the patient is turbid, uneven, thickened and so on, so some of the patients will have horizontal lines on the fingernail cover. 3. Psoriasis: the etiology of this disease is not clear, and it is currently believed to be caused by the combined effect of genetic and environmental factors. Common symptoms include pitting of nails, nail separation, hyperkeratosis under nails, etc. Therefore, psoriasis may be the cause of horizontal stripes on fingernail cover. It is recommended that patients with horizontal stripes on the fingernail cover go to the hospital, complete the relevant examination and clarify the specific cause.