A case of neonatal human herpesvirus 6 (HHV-6) B infection is presented. Although HHV-6 B was isolated from peripheral blood at the onset of the illness, a significant increase in viral antibody titers was not observed. The patient had a slight fever with generalized maculopapular skin rash and an increased number of atypical lymphocytes, which is quite different from the typical clinical features of exanthem subitum.