Cytomegalovirus (CMV) is the single most important infectious agent affecting recipients of organ transplants, with at least two-thirds of these patients having CMV infection 1-4 months after transplantation. Latently infected allografts are the major exogenous source of CMV infection in transplant recipients, although leukocyte-containing blood products can also transmit the virus. Three patterns of CMV infection are recognized: primary infection, reactivation infection, and superinfection. Primary infection has the greatest clinical impact. The clinical effects of CMV infection include infectious disease syndromes such as pneumonia and chorioretinitis; an immunosuppressed state that predisposes to potentially lethal opportunistic infection; and the initiation of a process that can result in allograft injury. Progress has been made in controlling CMV infection; hyperimmune anti-CMV globulin and certain antiviral drugs appear promising for prophylaxis, and the combination of hyperimmunoglobulin and ganciclovir appears promising for therapy.