Human herpesvirus 6B (HHV-6B) causes exanthema subitum in infants and is known to be mildly pathogenic. However, HHV-6B infection can induce febrile seizures in a high percentage of patients, and in rare cases, result in encephalitis. We detected higher levels of interleukin (IL)-1 beta and basic fibroblast growth factor (bFGF) in the cerebrospinal fluid (CFS) of patients with HHV-6B encephalitis when compared to those in patients with non-HHV-6B-induced febrile seizures. In vitro, IL-1 beta and bFGF enhanced HHV-6B gene expression in infected U373 astrocytes during the initial and maintenance phases of infection, respectively. These findings indicated that IL-1 beta and bFGF contribute to HHV-6B growth and the onset of encephalitis. (C) 2017 The Authors. Published by Elsevier Inc.