Purple patches on the arms consider lichen planus may be relatively large, the disease belongs to a complex etiology, occurring in the skin as well as the mucous membranes of subacute and chronic inflammatory skin diseases. Its etiology is mainly the following: first, cell-mediated immune response, direct immunofluorescence within the lesion can find flat moss-specific antigens to destroy the body’s tissues, leading to the appearance of purple patches. Second, genetic factors, about 10% of patients have a family history, and more sisters with the disease. Third, the infection factor, there are a few people due to the onset of infection, may have a certain relationship with the virus infection. Some patients can detect human papillomavirus HPV infection in the lesions, and some are associated with hepatitis C virus infection.