Brown vertical lines on the thumb nail are considered to be trauma, onychomycosis, onychomelanoma and so on. 1. Trauma: If the thumbnail area has suffered a violent impact, it will cause the capillaries under the nail to rupture and bleed, so it is manifested as brown vertical lines. 2. Nail nevus: Nail nevus is a kind of nevus, which is a kind of benign tumor and related to many factors, such as trauma, fungal infection, eczema-like lesions, etc. It is mainly manifested as nail black line. 3. Nail melanoma: this is a malignant tumor formed by the malignant transformation of melanocytes, if the tumor occurs under the nail, it will also be manifested as brown vertical lines on the thumbnail in the beginning, but the development of this disease is faster, which manifests itself as rapid enlargement, bulging, change in shape or color, and even itching and pain and other symptoms. Thumb nail with brown vertical lines may also be other causes, it is recommended to go to the hospital in time for consultation, by the doctor to clarify the cause after standardized treatment.