OBJECTIVES: After kidney transplantation, human BK polyomavirus (BKPyV) can induce a progressive disease, in three stages: viruria, viremia, and then nephropathy after a few months of viral replication. Therapeutic intervention is recommended when BKPyV is detected in the plasma. The objective of our study was to assess urinary BKPyV nucleic acid test as a predictor for developing viremia. METHODS: We first defined a viruria threshold based on 393 time-matched urine and plasma samples collected after kidney transplantation and then to validate this threshold, we followed-up a cohort of 236 kidney transplant patients. RESULTS: A BKPyV viruria threshold of 6.71 log10 copies/mL best discriminated between plasma-positive and plasma-negative patients (sensitivity: 90.9% (95%CI: 86.5-95); specificity: 90.3% (95%CI: 86.3-94.3); area under the curve: 0.953 (95%CI: 0.933-0.974). In the validation cohort, the risk of developing BKPyV viremia at one year was 16.5% (39/236) and rose to 90.7% (39/43) if BKPyV viruria remained above the threshold of 6.71 for more than one month. CONCLUSION: Sustained BKPyV viruria is a reliable, early marker of patients at high risk of developing BKPyV viremia. This marker should alert the clinician early, and thus allow timely therapeutic intervention.