Viral infections and clinical complications were studied during hemodialysis and after renal transplantation. Active cytomegalovirus infection developed in 96% of patients after renal transplantation; reactivation of herpes simplex, varicella-zoster, and Epstein-Barr viruses was found in 35%,24%, and 0% of patients, respectively. Cytomegalovirus viremia developed in 42% of patients an average of two months after renal transplantation, lasted 1.75 (±1.5) months (except in one patient with chronic viremia), and was followed by chronic viruria. Higher titers of infectious cytomegalovirus were found in the polymorphonuclear than in the mononuclear leukocyte fraction. Reactivation of a latent infection and, less likely, respiratory infection appear to be the most probable mechanisms of cytomegalovirus infection after renal transplantation. One to three months after transplant, cytomegalovirus infection may be related to fever, arthralgia, pneumonitis, and leukopenia; three to four months after transplant, the virus may be related to hepatitis; and 12–30 months after transplant, it may be related to retinitis in patients with chronic viremia. Although other causes of these complications are possible, herpes simplex virus, Epstein-Barr virus, varicella-zoster virus, measles virus, adenovirus, hepatitis B virus, and Toxoplasma gondii appear to be of lesser importance than cytomegalovirus in this respect.