To prospectively test whether the detection of three-dimensional, cast-like polyomavirus aggregates, termed Haufen, in voided urine samples can serve as an accurate biomarker of intra-renal disease, i.e. polyoma-BK-virus nephropathy (PVN). We want to correlate the detection of 'Haufen' with histologic findings made in renal biopsies as well as signs of polyomavirus activation, i.e. viremia and viruria. The prospective study is designed to further validate our retrospective findings (manuscript in press, J Am Soc Nephrology) and more specifically to correlate 'Haufen' shedding with the histologically confirmed course of PVN.