<h2>Abstract</h2><h3>Background:</h3> While ultraviolet (UV) light has been reported to contribute to the pathogenesis of systemic lupus erythematosus (SLE), the relationship between UV exposure and disease activity of SLE has not been fully investigated [1, 2]. <h3>Objectives:</h3> We aimed to investigate the effect of daily UV exposure on disease activity in patients with SLE. <h3>Methods:</h3> This retrospective study included SLE patients who visited a tertiary medical center in South Korea between January 2020 and January 2024 and were followed for at least 6 months. UV exposure was quantified using the UV index (UVI, range=0 - 11+), which was provided by the National Institute of Meteorological Sciences. The mean monthly UVI was used as a representative measure of UV exposure. Clinical data, including laboratory values related to SLE disease activity (white blood cell [WBC] count, lymphocyte count, neutrophil count, complement 3 [C3] and C4, erythrocyte sedimentation rate [ESR], and anti-double stranded DNA antibody [anti-dsDNA]), were retrieved from electric medical records. Relationships between UVI and lupus-related laboratory values or flare were analyzed using linear and logistic generalized estimating equations (GEE), accounting for repetitive measures within the study population. The β coefficients and odds ratios for UVI were adjusted for prespecified factors, including age, sex, season (winter or non-winter), and the dose of concomitant glucocorticoids. <h3>Results:</h3> A total of 423 patients were included in this study, with a mean (SD) age of 41.1 (14.7) years, and 368 (87.0%) were female. During the observation period, UVI peaked in summer (between May and July) and reached a nadir during winter (Figure 1). There were 129 cases of SLE flare during the observation period with an incidence rate of 8.47 per 100 person-years (95% CI: 7.07, 10.1). UVI was not associated with the occurrence of SLE flares (adjusted OR=1.04 [95% CI: 0.89, 1.21]). However, higher UVI was significantly associated with decreased WBC and neutrophil counts, as well as lower C3 levels (Table 1 ). <h3>Conclusion:</h3> Increased UV exposure is associated with worsening laboratory values but not with an increased risk of overt flares in patients with SLE. <h3>REFERENCES:</h3> [1] Skopelja-Gardner S, An J, Tai J, Tanaka L, Sun X, Hermanson P, et al. The early local and systemic Type I interferon responses to ultraviolet B light exposure are cGAS dependent. Sci Rep. 2020;10(1):7908. [2] Tanner TI, Agalliu I, Wahezi DM, Rubinstein T, Investigators CR. Relationship of ultraviolet light exposure and cutaneous and systemic disease activity in youth with childhood-onset systemic lupus: Results from the Childhood Arthritis and Rheumatology Research Alliance Registry. Res Sq. 2024. Figure 1Longitudinal trends of monthly UVI and flare occurrence <b>Table 1.</b> Effects of UV Exposure on Disease Activity and Flare Occurrence in SLE. <h3>Acknowledgements:</h3> <b>NIL</b>. <h3>Disclosure of Interests:</h3> <b>None declared</b>. © The Authors 2025. This abstract is an open access article published in Annals of Rheumatic Diseases under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Neither EULAR nor the publisher make any representation as to the accuracy of the content. The authors are solely responsible for the content in their abstract including accuracy of the facts, statements, results, conclusion, citing resources etc.