The B19 virus can persist in immunocompromised patients for several months and sometimes even years because of impaired immune response. Viremia in persistent and recurrent infection may range from very low to high titers and may be associated with chronic clinical manifestations, such as chronic anemia. Several recently developed techniques that quantify B19-DNA have improved laboratory diagnosis of the infection and can help guide the choice of treatment in persistent infections (i.e., intravenous immunoglobulin (IVIG) treatment vs immunosuppression reduction). Here we describe the development of a reliable internally controlled quantitative competitive (QC)- polymerase chain reaction (PCR) assay that measures B19-DNA load in serum samples by densitometric analysis of the amplification products for monitoring B19 infection in high-risk patients. A retrospective quantification of B19-DNA in the serum samples from 48 anemic transplanted patients by the QC-PCR assay we developed in our laboratory confirmed the presence of B19-DNA in 11 of 48 samples and showed a viral DNA load between 103 and 108 B19-DNA copies/mL depending on the patients' serostatus (the highest viral load was found in IgM-positive/IgG-negative patients, that is, in patients with active B19 infection at onset). The assay also confirmed B19-DNA negative patients. Our QC-PCR assay may be easily used in monitoring B19 prototype DNA load to follow persistent infections and to better understand the relation between active B19 infection and occurrence of anemia and to assess the efficacy of IVIG therapy or immunosuppression reduction in clearing the virus in high-risk patients.