AIM: To investigate predictive value of the systemic immune-inflammation index (SII) combined with the distal ureteral diameter ratio (UDR) for early breakthrough urinary tract infection (BT-UTI) in children with primary vesicoureteral reflux (VUR). METHODS: A retrospective analysis was conducted on 150 VUR patients admitted to ShangHai Children's Hospital from January 2021 to December 2023. Patients were divided into the early BT-UTI group (n = 52) and the non-earlyBT-UTI group (n = 98) based on whether early BT-UTI occurred. Univariate and binary logistics regression analyses were performed to identify factors influencing early BT-UTI in VUR children. Pearson correlation analysis was used to assess the relationship between variables, and receiver operating characteristic (ROC) curve was employed to evaluate the predictive value of SII combined with UDR for early BT-UTI in VUR children. RESULTS: Comparisons of age, gender, body mass index (BMI), neutrophils (NEU), lymphocytes (LYM), red blood cells (RBC), white blood cells (WBC), comorbidities, length of hospital stay, and caregiver education level showed no statistically significant differences (p > 0.05). Differences in reflux grade, UDR, SII, and platelets (PLT) between the groups were statistically significant (p < 0.05). According to Pearson linear correlation analysis, SII was negatively correlated with LYM, and positively correlated with PLT and NEU (r = 0.366 and 0.839, respectively; p < 0.05). UDR was unrelated to NEU and LYM, and positively correlated with PLT (r = 0.280, p < 0.05), and SII was positively correlated with UDR (r = 0.162, p < 0.05). Binary logistic regression analysis indicated that UDR and SII were factors influencing early BT-UTI in VUR children (OR = 1.171 and 5.306, respectively; 95% confidence interval (CI) = 1.099–1.249 and 2.841–9.912, respectively; p < 0.05). The combined variables had an area under the curve (AUC) of 0.832 with a standard error of 0.032 (95% CI: 0.768–0.895), Youden index = 0.53, sensitivity of 90.4%, and specificity of 62.2%. The combined index had an AUC closest to 1, indicating the highest predictive value. CONCLUSIONS: UDR combined with SII boasts a high predictive value for early BT-UTI in VUR children.