Polyomavirus BK virus (BKV) infection and nephropathy is a significant cause of allograft dysfunction in kidney transplantation. Clinical manifestation ranges from BK viremia and nephritis to renal dysfunction. It has been suggested that BK virus reactivation alone is not sufficient to cause BK viremia and nephropathy, thus a second hit is essential for kidney specific damage, such as an inflammatory reaction or ischemia. Critically ill postoperative/posttraumatic patients via the systemic inflammatory response syndrome (SIRS) and the compensatory antiinflammatory response syndrome (CARS) are at increased risk to develop organ dysfunctions, such as acute renal failure. CARS, reflecting postoperative/posttraumatic immunosuppression, may favor viral reactivation. However, prevalence of BK viremia in critically ill postoperative/posttraumatic patients has up to now not been systematically evaluated. Moreover, it is not known whether BK viremia is associated with a distinct biomarker pattern in these patients. Therefore, the present study is performed to clarify whether postoperative/posttraumatic immunosuppression is associated with BK viremia, and acute renal failure with BK virus reactivation, respectively.