Background: BK virus can frequently cause hemorrhagic cystitis in immunocompromised patients. There are limited data on BK virus infection in pediatric population. The purpose of this study was to investigate the effect of BK viruria on renal function in pediatric patients. Method: A retrospective review was performed in pediatric cancer patients and hematopoietic cell transplant (HCT) recipients who were tested for BK virus PCR in urine samples at Samsung Medical Center, Seoul, Korea from January 2003 to March 2013. Result: A total of 255 patients were tested for BK virus for 952 times. Among these, 111 patients had positive BK virus PCR results. The median age at BK virus test was 10.22 years (range, 0.48-21.96 years). Seventy-two patients were male (64.8%). Ninety-eight patients (88.3%) were HCT recipients. Underlying illnesses were acute leukemia (n=58, 52.3%), neuroblastoma (n=9, 8.1%), brain tumor (n=9, 8.1%), primary immunodeficiency (n=7, 6.3%), lymphoma (n=5, 4.5%), others (n=23, 20.7%). The patients were divided into two groups based on peak virus titer in urine. Virus titer of 10,000 copies/mL was chosen as cutoff value between high and low groups. There were 86 patients in high group and 25 patients in low group. The median peak virus titers were 1.32 x109 copies/mL (95% CI, 7.37x109 - 1.87x1010 copies/mL) in high group and 950 copies/mL (95% CI, 740.19 - 2488.9 copies/mL) in low group, respectively ( P0.0001). Median serum creatinine level before and after 2 weeks around the time of initial positive BK virus titer was 0.72 mg/dL (95% CI, 0.64 to 0.79 mg/dL) in high group and 0.63 mg/dL (95% CI, 0.41 to 0.84) in low group, respectively. Median serum creatinine level in high group was significantly higher than that in low group ( P= 0.0229, Mann-Whitney test). Seven patients received continuous renal replacement therapy (CRRT) within 60 days after first positive BK virus PCR results in high group while none required CRRT in low group ( P=0.3463, RR 1.316, 95% CI, 1.182 to 1.467). Conclusion: Patients with high BK virus titer above 10,000 copies/mL in urine had increased creatinine levels. A further study on long-term effect of BK viruria on renal function in these growing children would be needed.