||The BK virus, a member of the polyomavirus family, has become a serious problem in transplantation, especially for renal transplant recipients. The incidence of BK viremia (and viruria) post-transplantation has been rising (and may be approaching 10%). The pathogenesis is unclear but may be related to newer immunosuppressive drugs and the fact that BK virus may be latent in uroepithelial cells. BK renal tubulointerstitial disease is associated with significant graft dysfunction in most patients. Post-transplant screening for BK virus in asymptomatic recipients is becoming increasingly common. Also, the positive predictive value of detecting high levels of BK virus may avert the need for biopsy in patients with suspected early BK renal disease. Prompt reduction in immunosuppression and the addition of anti-viral drugs may prevent graft loss. This lab-developed test targets a conserved region of the viral VP1 gene, and is validated for both plasma and urine samples.