Sixty‐five renal allograft recipients on immunosuppressive therapy were examined for BK and JC viruria using Southern blot hybridization. The incidence and degree of BK and JC viruria were compared between this population (group RTR) and, an age‐ and sex‐matched population of non‐immunosuppressed individuals (group CTR). In the results, the incidence of BK viruria was significantly increased in the RTR group compared with the CTR group, while that of JC viruria was similar in the 2 groups. The proportion with a high level of JC viruria, however, was greater in the RTR group compared with the CTR group. Additionally, it was also demonstrated that the incidence of both BK and JC viruria was not affected by the characteristics of renal transplant recipients, such as differences in the donor source (living‐related vs cadaveric), type of immunosuppressive agents used, or the time post‐tran