Rapid, specific and sensitive methods are essential for early detection of CMV infection in patients after marrow transplantation. Thus, in a prospective study, PCR from whole blood, plasma-PCR, pp65-antigenemia and virus culture were used and compared in 20 consecutive marrow transplant recipients for early diagnosis of CMV infection and monitoring of antiviral therapy. Moreover, semi-quantification of the viral load in blood samples by PCR from whole blood or plasma and pp65-antigenemia was performed. Fifteen out of 20 patients were found to be CMV positive by PCR from whole blood, plasma-PCR and pp65-antigenemia, whereas only 9/20 developed culture-proven viremia and/or viruria. PCR from whole blood, plasma-PCR and pp65-antigenemia revealed identical results in 96 and discordant results in 13 of 109 blood samples (P < 0.01). The efficacy of antiviral therapy was monitored by semi-quantitative scoring of pp65-antigen-positive leukocytes and/or CMV-DNA levels in blood and plasma samples. Twelve of 13 patients were found to be CMV negative by all methods after 14 days of ganciclovir therapy. A good correlation of the semi-quantitative evaluation of the three assays was demonstrated. Thus, all three highly sensitive assays seem to be suitable for screening patients at risk for CMV infection and monitoring the efficacy of antiviral therapy.