CMV infection is a major cause of morbidity and mortality following renal transplantation. Clinical diagnosis is difficult, and rapid and sensitive diagnostic methods are needed since antiviral therapy is available. The determination of the presence of viral transcripts is considered a direct marker of HCMV replication in vivo. In particular, it seems that the expression of late transcripts might better reflect active HCMV replication, dissemination and disease, and should cease upon effective blockage of viral polymerase by antiviral agents, such as gancyclovir. The unspliced pp67-mRNA can be specifically amplified using nucleic acid sequence-based amplification (NASBA) in a background of DNA. In the present study blood samples of forty-two renal transplant patients with active HCMV infection, demonstrated by pp65-antigenaemia, were investigated to detect pp67-mRNA using the NASBA technique. Thirty-one pp65-antigenaemia positive patients resulted NASBA positive (73.8%) also in case of very low levels of antigenaemia; in 9/42 (21.4%) pp67-m-RNA was detected between 6 and 15 days before antigenaemia. Our results indicate that pp67-mRNA NASBA is a useful tool for the early diagnosis of active HCMV infection and for starting and modulating antiviral therapy, in addition to quantitative techniques such as antigenaemia, in renal transplant patients.