The viral infections with greatest impact on the renal transplant recipient are those due to cytomegalovirus, Epstein-Barr virus, and the two hepatitis viruses, hepatitis B and C. All of these are modulated by the administered immunosuppressive therapy, and all have both direct and indirect effects on the transplant patient. The direct effects are the infectious disease clinical syndromes that are produced (fever and malaise, pneumonia, hepatitis, and so forth). The indirect effects are several--all of these viruses contribute to the patient's net state of immunosuppression, predisposing him or her to the development of opportunistic superinfection with a variety of pathogens. In addition, both Epstein-Barr virus and hepatitis B virus have been clearly linked to the development of certain malignancies (lymphoproliferative disease and hepatocellular carcinoma, respectively). Finally, cytomegalovirus has been linked to allograft injury. Although antiviral strategies effective for cytomegalovirus and Epstein-Barr virus infection are being developed, similar programs are not yet available for the hepatitis viruses.