Human Cytomegalovirus (HCMV) transcripts (including UL21.5 mRNA) have been found to be packaged in virions and their detection in plasma may indicate the presence of infectious viral particles. Objective of this study was to verify whether UL21.5 mRNA detected in the plasma was indeed encapsulated in viral particles, representing an indirect marker of active replication. To distinguish between virion-packaged and free-floating RNA, plasma samples from 22 immunocompromised patients were tested before and after ribonuclease (RNAse) digestion. UL21.5 mRNA was detected 1-2 weeks prior to preemptive therapy administration in 20 episodes (from 18 patients) of clinically significant DNAemia, while it was undetectable in three of the four patients with transient, self-resolving DNAemia. After RNAse digestion, UL21.5 mRNA was still detectable, with a median reduction of 0.1 (IQ range 0-0.3) Log ₁₀. Concentrations of UL21.5 mRNA in plasma correlated significantly with HCMV DNA in whole blood or plasma (R=0.67), and 75% of samples positive for UL21.5 mRNA had HCMV DNA concentrations above 10 ⁴ copies/ml blood or 10 ³ copies/ml plasma. Moreover, UL21.5 mRNA was positive in patients who developed HCMV infection resistant to letermovir or maribavir, whereas it was undetectable in plasma of patients with transient self-resolving DNAemia blips during letermovir prophylaxis (not associated with drug-resistance). HCMV UL21.5 mRNA in plasma is virion-associated and represents a marker for productive HCMV infection. The determination of UL21.5 mRNA could improve current strategies for the management of HCMV infection in immunocompromised patients.