BK virus-associated nephropathy (BKVN) has become recognized as an important cause of allograft dysfunction among transplant recipients. Despite reduction in immunosuppression, 30%-40% of recipients progress to allograft loss. Cidofovir is an antiviral agent that has been proposed for treatment of BKVN. We describe the clinical course, renal function, and blood viral measurement in 6 renal transplant recipients with BKVN who were treated with low doses of cidofovir. Administration of cidofovir was associated with clearance of BK virus DNA from blood and stabilization of renal function in 5 cases. These data suggest that cidofovir may be useful as adjuvant therapy for BKVN.