BK infection is an important cause of graft dysfunction and graft loss after renal transplantation. It has been widely accepted that emergence of BK virus correlates with the more potent immunosuppressive agents used to lower acute rejection rates. In contrast to other opportunistic infections after transplantation, for which routine prophylactic agents are administered, there is no effective agent for the prevention of BK infection. Some data, however, suggests that quinolone antibiotics such as ciprofloxacin may have activity against BK virus. This has led us to investigate whether routine, short-term ciprofloxacin administration post-transplant can lower the incidence of BK infection.