Polyoma BK virus (BKV) remains dormant in the urinary tract and circulating leucocytes and becomes reactivated during immunosuppression. BK viruria is prevalent in renal allograft recipients and BK viremia may be related to nephropathy and allograft rejection. How BK viruria and viremia are related in renal allograft patients is undefined. In this study, BKV copies in paired urine and serum samples of renal allograft recipients were measured by a real time quantitative polymerase chain reaction (Q-PCR) to test the hypothesis that their quantitative relationship might help to delineate viral reactivation patterns in these tissues. Urine and plasma samples from 44 renal allograft recipients with stable graft function were collected during outpatient follow-up and the genome copies of BKV were determined by Q-PCR. All patients showed quantifiable viremia and two groups of patients were identified: one group of patients (n=35) showed low viral load (median: 270/ml, range: 108-1000/ml) and the other group (n=9) with high viral load (median: 5x10(4)/ml, range: 2x10(4)-6x10(4)/ml). The corresponding median levels of viruria were 2000 and 900 ml. BK viremia and viruria were not related quantitatively. BK viremia/viruria were also not related to age, immunosuppression, time and source of renal grafts and serum creatinine levels. The absence of a quantitative relationship between BK viremia and viruria may reflect independent BKV reactivation in different tissues during immunosuppression.