Previously we reported that the rate of mild to moderate infections and upper respiratory infections in infants who undergo transplantation under 1 year of age was the same as that of the general population. Despite this, serious infections continue to be one of the major complications in the first 3 to 4 months after heart transplantation. Among newborns and infants at Loma Linda University Medical Center, there have been 35 deaths from various causes: six (17%) were the result of infectious causes, three were in the early perioperative period, and three were late complications. There were no deaths caused directly by cytomegalovirus. From 1989 to 1992, 128 under 1 year of age underwent transplantation. Of these, 65 had at least one episode of serious infection ranging from bacterial meningitis to viral pneumonia. Of these infants, 19 had cytomegalovirus infections, with the vast majority having symptoms in the first 2 to 3 months after transplantation. Eight infants had Pneumocystis pneumonia and were treated successfully. The risk of cytomegalovirus is highest in the first 4 to 8 weeks after transplantation and in a seronegative recipient who has received an organ from a seropositive donor. Currently, our protocol to reduce the likelihood of active disease includes the use of intravenous immunoglobulin immediately after transplantation and during rejection episodes that are treated with aggressive immunosuppression and the use of oral acyclovir for the first 3 months after transplantation. If active disease develops, ganciclovir is initiated.(ABSTRACT TRUNCATED AT 250 WORDS)