The DNA of herpesviruses such as Epstein-Barr virus (EBV), cytomegalovirus (CMV), human herpesvirus-6 (HHV6), and human herpesvirus-7 (HHV7) has been detected in the serum of patients with primary infection or with immunosuppression. However, it is unknown how frequently herpesvirus DNA can be detected in the serum of immunocompetent children, or whether the detection of herpesvirus DNA indicates an active infection or virus-related diseases. Using a real-time polymerase chain reaction assay, attempts were made to detect herpesvirus DNA in the serum of 176 ambulatory children who visited a hospital for various reasons. EBV was detected in 4 (2.2%), HHV6 in 4 (2.2%), and HHV7 in 2 (1.1%) of 176 children, but CMV was not detected. Of the 10 positive patients, only 4 were considered, by virtue of clinical and serological characteristics, to have primary infections. The other 4 positive patients had other infections, such as mycoplasma and salmonella. Although herpesvirus DNA could be detected in the serum of immunocompetent children, there was not always a relationship between clinical manifestations and the detection of virus DNA. When herpesvirus DNA is detected in the serum, a careful interpretation is necessary to diagnose a primary infection or a virus-associated disease.