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Objective: Systemic lupus erythematosus is a multisystem disease with many clinical variations, including renal involvement. Our aim was to determine whether lupus nephritis (LN) has a specific seasonality., Methods: Reports of renal biopsies performed from 1990 to 2002 were reviewed. Three hundred and seventy-three patients with class II, III, IV, and V LN were identified. Using the modified WHO classification of LN, diagnoses were tabulated and the seasonality (season of diagnosis) of LN was statistically analyzed., Results: Class IV LN was detected in 179 patients (48%), class II in 63 patients (16.9%), class III in 73 patients (19.57%), and class V in 74 patients (19.9%). No difference could be detected in the number of patients diagnosed in each season when all 373 patients were analyzed as one group. The number of patients with class IV LN was higher during summer and fall than during the winter and spring. In contrast, a higher number of patients with class V LN were observed during the winter and spring seasons than during the summer and fall seasons. The percentage of patients with class V LN was significantly higher during winter and spring than during summer and fall. A similar, though non-significant, trend was seen for class III LN. A striking parallelism was found between the month of occurrence of class III and class V LN. The monthly distribution of the percentage of patients in each month with class III and V LN showed a significant correlation. The monthly distribution of patients with class IV LN was different from those with either class III or V LN., Conclusion: We found that the prevalence of class V LN was significantly higher and that of class III LN non-significantly higher in winter and spring. Parallelism between the monthly occurrences of class III and class V may suggest a common trigger. Analysis of the seasonality of LN may contribute to the understanding of the pathogenesis of LN, which may be multifactorial, as the different classes of LN represent different types of glomerular injury. Further studies are needed to clarify this potentially important observation.