BK virus is a small nonenveloped, double-stranded DNA virus from the polyomaviridae family. It was originally isolated from a renal transplantation patient with ureteric stenosis [1]. BK virus is widespread in the general population, with 90% of the population positive by adulthood [2]. Primary infection causes mild respiratory illness and then the virus enters a latent phase, particularly in the urogenital tract (kidneys, urinary bladder, prostate) and lymphoid tissues (tonsils). Periodically, BK virus reactivates and sheds into the urine of 5% to 10% of healthy hosts without symptoms [3]. However, BK virus reactivation is common in immune-compromised patients, and it causes a variety of clinical manifestations, including asymptomatic hematuria, hemorrhagic cystitis, ureteral stenosis, and interstitial nephritis [4-6]. Hemorrhagic cystitis (HC) is the most prevalent complication among recipients of allogeneic hematopoietic stem cell transplantation (HSCT), with an incidence ranging from 10% to 25%, and it is associated with significant morbidity and mortality [7]. In 1986, Arthur et al. [8] published a series of 53 patients showing that HC occurred 4 times more frequently in patients who excreted BK virus than in those who did not. It is now generally believed that BK virus infection during HSCT is due to reactivation of latent virus, although new or reinfection has also been postulated. Many studies later identified a quantitative association between BK virus and late HC, and the studies have noted that BK viruria preceded or coincided with the onset of disease [4,8]. However, 40% to 50% of HSCT patients with persistent BK viruria remained free of HC, indicating that other factors, such as conditioning regimen intensity, acute GVHD, severity of immune suppression, unrelated donor, and presence of anti-BK virus IgG before transplantation, contributed to the development of HC [4,5]. In this issue of Biology of Bone Marrow Transplantation, Laskin et al. [9] study the correlation between BK PCR and HC in a previously completed prospective cohort of 88 children (ages, 1.8 to 15.2 years) who received HSCT at single transplantation center. The study collected plasma weekly from day 0 until day þ100. Plasma BK PCR with at least 1 measurement between days 0 to 14, 15 to 85, and 100 4 after HSCT established 4 cut-off values for BK PCR as follows: 0; 1 to 9999; 10,000 to 99,9999; and 100,000 PCR copies/ mL. The BK PCR in urine was measured only in symptomatic