The risk of opportunistic infection in the renal transplant patient is due to an interaction between two major factors: the epidemiologic exposures (particularly within the hospital environment) and the net state of immunosuppression. The net state of immunosuppression is determined by the nature, dose, and duration of the immunosuppressive therapy being administered; the presence or absence of granulocytopenia and technical factors that could compromise the primary mucocutaneous barriers to infection; such metabolic factors as uremia, hyperglycemia, and the state of nutrition; and, finally, the immunomodulating effects of such viruses as CMV, the hepatitis viruses, and HIV. The major types of opportunistic infection to which the renal transplant patient is susceptible are the following: the viruses of the herpes group and papovaviruses; bacteria such as L monocytogenes, N asteroides, and Legionella; such fungi as Candida, Aspergillus, C neoformans, and the Mucoraceae; and protozoans such as P carinii, S stercoralis, and T gondii.