The etiologic agent of severe acute respiratory syndrome (SARS) has been identified as a new type of coronavirus, known as SARS-coronavirus (SARS-CoV). Although the SARS epidemic has subsided, many authorities, including the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC), have warned of the possible re-emergence of this highly infectious disease. Although antibody-based diagnosis of SARS has been demonstrated to be a reliable proof of SARS infection, it is not sensitive enough for detection during the early phase of the disease. To date, based on the publicly released full genomic sequences of SARS-CoV, various molecular detection methods based on reverse-transcription polymerase chain reaction (RT-PCR) have been developed. Although most of the assays have initially been focused on RNA extracted from nasopharyngeal aspirates, urine, and stools, several of the more recently developed assays have been based on the analysis of RNA extracted from plasma and serum. Such assays allow the more standardized quantitative expression of viral loads and are potentially useful for early SARS diagnosis. In this chapter, two real-time quantitative RT-PCR systems for the quantification of SARS-CoV RNA in serum are discussed. The two RT-PCR systems, one aimed toward the nucleocapsid region and the other toward the polymerase region of the virus genome, have a detection rate of up to 80% during the first week of illness. These quantitative systems are potentially useful for the early diagnosis of SARS and can also provide viral load information that might assist clinicians in making a prognostic evaluation of an infected individual.