Abstract: Objective To investigate the prognostic value of systemic immune-inflammation index (SII) in patients with IgA nephropathy.Methods Retrospective analysis was conducted on baseline laboratory and pathological examination results of patients who underwent their first renal biopsy and were pathologically diagnosed with primary IgA nephropathy at the Affiliated Hospital of Xuzhou Medical University from January 2017 to December 2021. The optimal cutoff value for SII was determined using the ROC curve. Accordingly, the patients were divided into two groups: a high SII group and a low SII group. Their clinical data were compared. Kaplan-Meier curves and multivariable Cox models were used to assess the relationship between SII and prognosis in patients with IgA nephropathy.Results A total of 143 patients with IgA nephropathy were included in this study, with a median follow-up time of 31 (22,38) months. SII was found to be closely associated with the pathological grades of IgA nephropathy, and serum SII levels were significantly higher in female patients than in male patients (P<0.05). Cox regression analysis showed that segmental glomerulosclerosis, platelet to lymphocyte ratio (PLR), 24 hour proteinuria content, and SII were independent risk factors for IgA nephropathy. Kaplan-Meier curves indicated that patients in the high SII group were more likely to experience poor renal outcomes compared with those in the low SII group (P<0.05).Conclusions SII is closely related to clinical risk factors and pathological grades in primary IgA nephropathy, with higher SII levels associated with worse patient prognosis. SII is an independent prognostic factor for poor prognosis in IgA nephropathy.