Blood samples from 132 consecutive hematopoietic stem cell transplant recipients were obtained and tested weekly for BK virus DNA by use of quantitative real-time PCR. Forty-four patients (33%) developed BK viremia at a median of 41 days (range, 9-91 days) after transplantation. Patients with hemorrhagic cystitis that occurred after platelet engraftment had higher levels of viremia than did patients without hemorrhagic cystitis (median, 9.7x10(3) vs. 0 copies/mL; P=.008) and patients with hemorrhagic cystitis that occurred before platelet engraftment (median, 9.7x10(3) vs. 0 copies/mL; P=.0006). BK viremia also was strongly associated with postengraftment hemorrhagic cystitis in a time-dependent analysis (P=.004).