BK virus (BKV) and JC virus (JCV) infections were evaluated in a serological study of 496 renal transplant recipients and their donors. A seropositive donor increased the rate of primary and reactivation infections with BKV and of primary infections with JCV. BKV infection rates were not influenced by the source of the renal allograft (cadaver versus living related donor); however, primary JCV infections occurred more often in recipients of seropositive cadaveric kidneys. Reactivated JCV infections occurred less frequently in patients treated with antilymphocyte preparation. BKV and JCV infections in renal transplant recipients may be caused either by reactivation of the recipient's latent virus or by virus from the donor kidney. These infections are, however, not associated with adverse outcome (death, high serum creatinine level, or loss of renal function) in the recipient in the early post-transplant period.