Background: The etiologic agent of severe acute respiratory syndrome (SARS) is a recently identified, positive singlestranded RNA (ssRNA) coronavirus (SARS-CoV). Little is known about the dynamic changes of the viral replicative form in SARS cases. Objectives: Evaluate whether SARS-CoV can infect and replicate in peripheral blood mononuclear cells (PBMCs) of infected persons and reveal any dynamic changes to the virus during the course of the disease. Study design: Peripheral blood mononuclear cells collected from SARS cases infected by the same infectious source were tested for both negative-stranded RNA (minus-RNA, "replicative intermediates") and positivestranded RNA (genomic RNA) of SARS-CoV during the course of hospitalization by reverse transcription-polymerase chain reaction (RT-PCR). Results: SARS-CoV minus-RNA was detected in PBMCs from SARS patients. The viral replicative forms in PBMCs were detectable during a period of 6 days post-onset of the disease, while the plus-RNA were detectable for a longer period (8/12 days post-onset). Conclusions: SARS-coronavirus can infect and replicate within PBMCs of SARS patients, but viral replication in PBMCs seems subject to self-limitation.