Aim: BK virus (BKV)-associated hemorrhagic cystitis (HC) is a common complication inpatients after hematopoietic stem cell transplantation (HSCT). The aim of this study was toinvestigate the incidence of BKV infection in pediatric patients receiving HSCT.Material and Methods: Total of 51 patients aged between 16 months and 16 years old andfollowed up between October 2015 and September 2017 were included in the study. Thepatients were monitored by quantitative real-time polymerase chain reaction (AnatoliaGeneworks, Turkey) test for the detection of BKV DNA in urine and blood.Results: Of patients, 46 received allogeneic HSCT and 5 autologous HSCT. BKV DNApositivity was detected in urine and/or blood of total 27 (52.9%) patients in whom 26 (56.5%)of 46 patients with allogeneic transplantation, and 1 (20.0%) of 5 patients with autologoustransplantation. BKV viral load in urine >107copies/ml required for preemptive treatment wasdetected in 12 (26.1%) of 46 patients received allogeneic HSCT. The development of HC wasprevented in 9 (75.0%) of the 12 patients given preemptive treatment, while 3 (25.0%) casesdeveloped HC and cured by treatment. BKV viruria was detected >109copies/ml in two weeksbefore the onset of HC and was accepted as a prognostic indicator for predictive diagnosis ofHC. BKV viremia was found >104copies/ml in 1 patient within two weeks before the onset ofcystitis.Conclusion: Screening for BKV infection, especially BKV viruria in HSCT patients, isrecommended for the predictive diagnosis of HC in patients at high risk.