BK polyomavirus-associated nephropathy (BKPVAN) is a major cause of renal failure early after kidney transplantation. The present study reports the preliminary results of prospective monitoring including a preemptive strategy for BKPVAN during the first year after kidney transplantation.We monitored BK virus DNA in blood at months 1, 2, 3, 6, 9, and 12 among 92 subjects who received induction therapy (basiliximab or antithymocyte globulin), and maintenance immunosuppression with prednisone, mycophenolate mofetil, and tacrolimus. Patients with two or more consecutive measurements of viral load >10(4) copies/mL were treated with a stepwise approach including dose reduction or discontinuation of mycophenolate mofetil eventually followed by reduction of tacrolimus and introduction of leflunomide.Within 1 year, seven (7%) patients displayed sustained BK viremia at a median of 92 days after transplantation. Among 68 patients who underwent a renal allograft biopsy, seven were diagnosed as BKPVAN at a median of 15 weeks after transplantation. The diagnosis was achieved by a surveillance biopsy in four patients with stable renal function. BKPVAN was preceded by asymptomatic viremia except for two cases in whom BK viremia occurred at 6 or 11 months, after the histological diagnosis. At 12 months, six patients had cleared their viremia. Serum creatinine levels had stabilized in six recipients with BKPVAN estimated renal function was 43.7 ± 16.3 mL/min in patients with viremia and/or BKPVAN versus 61.3 ± 20.1 mL/min among patients who never became viremic (P = .03). None of the patients with viremia and/or BKPVAN lost the allograft.BKPVAN may occur early after kidney transplantation, at a low or undetectable viremia or at some weeks after the first positive viremia. Intensive monitoring during the first 4 months after transplantation together with early protocol biopsies or interventions prompted by BK viremia may optimize BKPVAN diagnosis at a subclinical stage, thus avoiding renal dysfunction.