Background: To examine if there was a monthly variation in HLA-B27-associated uveitis., Methods: Retrospective review of all individuals presenting to a single centre with HLA-B27-associated uveitis from 2009 to 2020., Results: 577 participants with HLA-B27-associated uveitis were included. The median age at presentation was 40.4 years (IQR 31.1-51.4) and 356 (61.7%) were male. 141 (24.4%) participants were diagnosed with ankylosing spondylitis. The incidence of HLA-B27 uveitis was greatest in August (winter) and lowest in March (autumn). On univariate analysis, there was an inverse relationship between HLA-B27-associated uveitis incidence and mean air temperature (β-2.704, p < 0.001) and sunlight hours (β-0.221, p = 0.003), and a positive relationship between uveitis incidence and rainfall (β 0.324, p = 0.006) and humidity (β 1.741, p = 0.006). On multivariate analysis, there was an inverse relationship with temperature (β-4.846, p = 0.002) and a positive association with humidity (β 2.033, p = 0.008). On multivariate analysis, the impact of rainfall shifted to negative (β-0.446, p = 0.017) and sunlight hours lost significance. The monthly effect was more pronounced for non-Caucasian ethnicities and for those without ankylosing spondylitis., Conclusion: In New Zealand, HLA-B27-associated uveitis episodes are influenced by monthly changes with higher incidence in winter months., (© 2025 The Author(s). Clinical & Experimental Ophthalmology published by John Wiley & Sons Australia, Ltd on behalf of Royal Australian and New Zealand College of Ophthalmologists.)