Background: Urinary tract infection (UTI) is a common bacterial infection in childhood, and vitamin D deficiency is a potential risk factor. This study compared serum 25-hydroxyvitamin D levels in children with uncomplicated UTI and healthy children.

Methods: This case-control study of 96 children was conducted in Birjand during 2024–2025. Serum 25-hydroxyvitamin D levels were measured using a arm-vein blood samples. To diagnose UTI, patients underwent urine analysis (U/A) and urine culture (U/C). Data were collected through a checklist and analyzed using SPSS version 23 with Mann-Whitney U test , Kruskal-Wallis H test, and Chi-square test at a significance level of <0.05.

Results: Most were girls (91.7%) and older than 5 years (62.5%). There was no significant difference between the two groups in age, sex, height, weight, place of residence, or breastfeeding history (p>0.05). The median (Q1–Q3) serum 25-hydroxyvitamin D level was 18.30 (12.90–28.15) ng/ml in the case group and 20.1 (14.925–28.525) ng/ml in the control group; this difference was not significant (p=0.47), and the effect size was very small (r=0.07). There was also no significant difference in the prevalence of vitamin D deficiency between the groups (60.4% vs. 50.0%, p=0.24).

Conclusion: This study found no significant association between serum vitamin D levels and first-episode uncomplicated UTI. The high background prevalence of vitamin D deficiency, restriction to first-episode uncomplicated infections, limited sample size, and small effect size (r=0.07) may have contributed to the lack of a detectable association. These findings do not support a specific role for vitamin D in first-episode UTI risk; however, the high prevalence of deficiency remains clinically important for general pediatric health monitoring.