Most reports now highlight the dangers of sun exposure in patients with lupus. About 1/3 of patients with lupus develop a bright red rash on the cheeks or other exposed areas after sun exposure or an aggravation of an existing rash, which is called “photosensitivity”. After a hot summer harvest or after sunbathing at the beach, the rash reappears and proteinuria appears in some lupus patients, indicating a relapse or worsening of the disease. A lupus patient abroad had her disease worsened after her annual summer vacation at the beach, so she wrote a book about her illness and treatment experience called “The Sun is My Enemy”. The reason for the relapse or exacerbation of the disease is that ultraviolet light from the sun denatures the deoxyribonucleic acid (DNA) in the skin, producing antigenic thymine dimers, and thus the body produces anti-DNA antibodies. The combination of antigen (DNA) and anti-DNA antibodies, with the involvement of complement, forms immune complexes that are deposited in blood vessels and tissues, causing damage. In addition, UV light has been found to cause skin keratinocytes to secrete interleukin-1, which in turn stimulates B lymphocytes to produce growth factors and differentiation factors, causing B lymphocytes to proliferate and produce large amounts of antibodies; UV light also weakens the body’s processing of immune complexes, which can cause exacerbation of the disease. Patients with lupus erythematosus should avoid long-term exposure to strong sunlight, let alone working under direct UV radiation. In daily life, patients with lupus erythematosus should not swim or sunbathe in beach baths. Patients should wear straw hats, long-sleeved clothes and sunscreen for outdoor work in summer. New and effective sunscreens can be used to protect the skin, but their protective strength should not be lower than SPF15 (solar protection factor), which means that people who use sunscreens are 15 times more resistant to the sun than those who do not. Sunscreens should block both UV-A and UV-B. However, the average patient with SLE or discoid lupus can get 10 to 15 minutes of sun exposure per day at midday without sunscreen, and the degree of protection needed depends largely on the presence or absence of a rash, the location of the rash, and the presence of previous photosensitivity. Patients who have never had a rash do not have to fear the sun; they do not have to wear an umbrella when they go out. Some patients do not have photosensitivity even after prolonged sun exposure, but others may also develop photosensitivity in winter under strong sunlight reflections in the snow, which means that the degree of sensitivity to sunlight varies from person to person. Excessive exposure to sunlight, such as sunbathing, sitting too long at the pool, fishing or resting on the beach, even in the shade, may cause mild flare-ups of the disease. Patients under treatment should also avoid these exposures. Sunlight reflecting off the water, beach or snow should also be protected, as it can cause damage that is not readily apparent to the patient. In conclusion, patients with lupus erythematosus should not be exposed to direct sunlight for more than 15 minutes in high summer, regardless of the presence of a rash.