The aim of this prospective study was to investigate the role of HHV-6 infection in children with acute onset of liver failure using real-time quantitative PCR. Twenty-three children (median age, 24 months) were included: 6 cases of fulminant hepatic failure of undetermined cause (group 1); 4 cases of fulminant hepatic failure of recognized cause (group 2); 3 cases of acute decompensation of chronic liver disease (group 3); and 10 cases of chronic liver disease (group 4). HHV-6 genomic DNA was detected and quantified using real-time PCR in plasma and livers obtained at the time of transplantation. HHV6-DNA detection rate was significantly higher among groups 1, 2, and 3 compared to group 4 (76.9% vs. 20% P=0.02). Viral loads ranged from 6 to 32,500 copies/106 cells. Significantly higher viral loads were found in 4 of 9 children with acute onset of liver failure of unknown origin (group 1, n = 3; group 3, n= 1) and 1 child with fulminant autoimmune hepatitis (group 2) (P=0.03). These results strongly support the hypothesis that HHV-6 may cause fulminant hepatic failure and acute decompensation of chronic liver disease in children. Nevertheless, a threshold viral load value still remains to be determined. J.Med. Virol. 80:1051-1057, 2008. © 2008 Wiley-Liss, Inc.