The well-calculated and early application of specific antiviral drugs in cytomegalovirus (CMV) infections depends on sufficient virological diagnosis. Many authors prefer the detection of CMV DNA by the polymerase chain reaction (PCR) for this purpose. In our study in 41 renal transplant patients PCR yielded no additional information of clinical relevance compared to the already established pp65 antigenemia assay. However, neither of the two techniques allowed the prediction of symptomatic infections as also asymptomatic infections yielded positive results. In contrast, CMV mRNA was demonstrated by PCR mainly in symptomatic infections. We conclude, that mRNA amplification possibly is able to predict symptomatic CMV infections and will thereby allow the well-calculated application of e.g. ganciclovir.