A prospective virologic follow-up of solid organ transplant patients was designed to determine the usefulness of antigenemia and viremia as virologic markers for the diagnosis of cytomegalovirus (CMV) infections, and also for monitoring CMV disease and therapy control. A total of 629 blood samples from 127 patients (60 liver, 47 kidney, and 20 heart transplant recipients) were studied by tube and shell vial cultures, and by antigenemia assay. This later was carried out by an indirect immunofluorescent assay method for formalin-fixed cytospin slides containing 2 x 10(5) leukocytes, using a monoclonal antibody directed against the CMV pp65 antigen. CMV was detected by at least one of the three methods in 238 specimens (37.8%) from a total of 63 patients. The antigenemia assay was positive in 215 (90.3% of positive samples). A total of 94 samples were detected only by this marker, which occurred either in samples with low positive counts (70.2% with antigenemia counts < 10 positive cells/10(5) leukocytes) or in specimens from treated patients. There were 30 episodes of CMV disease in 23 patients. Antigenemia was positive in all these episodes, 27 of them with counts > 20 positive cells/10(5) leukocytes. With this cut-off, positive and negative predictive values for symptomatic CMV infection were 100% and 97.2%, respectively. The antigenemia assay is a rapid, sensitive, specific, and early marker of CMV infection in transplantees. Cultures became negative with antiviral therapy while remaining antigenemia detectable. There was an association between highest quantitative antigenemia test results and clinical symptoms in our patients. In its quantitative version, the assay is useful to detect symptomatic infection and appears to be a helpful tool in managing patients at risk and in guiding antiviral therapy.