BK virus infections are very frequent during months following a kidney transplantation: a viral reactivation is observed for almost 50% of patients during first year. This reactivation leads to a viremia for 10 to 15% of patient during this same period. The most frequent complication is interstitial nephritis for 2 to 8% of patients (27 patients representing 2.7% during 6 years in Nantes).