In a significant number of patients, the etiology of fulminant hepatic failure (FHF) is unknown. To determine whether hepatitis B virus (HBV) and hepatitis C virus (HCV) play a role in patients without serologic markers of HBV and HCV infection, the authors examined tissue samples from 15 liver explants with massive hepatic necrosis for the presence of viral sequences by the polymerase chain reaction (PCR). The specimens were derived from nine patients with FHF of unknown etiology; two with serum hepatitis B surface antigen (HBsAg); two with antibodies to HCV; one with antibodies to hepatitis A virus (HAV) and anti-HBc of the IgM class; and one with isoniazid toxicity. Nucleic acids were extracted from frozen liver samples. RNA was used as a template for reverse transcription, followed by double PCR with nested primers for the 5'-untranslated region of HCV. DNA was tested by single PCR for S gene sequences of HBV. Hepatitis B virus sequences were detected in the specimens of the two HBsAg positive patients, the anti-HAV/anti-HBc positive patient, and three of nine patients with FHF of unknown etiology. Hepatitis C virus sequences were present in the explant of one patient with FHF of unknown etiology, but not in the two patients with antibodies to HCV. In two specimens with molecular findings of HBV infection (1 from a patient with serum HBsAg and 1 without), there was immunohistochemical evidence of coinfection or superinfection with hepatitis delta virus (HDV). In conclusion, in this patient population, HBV, alone or with HDV or HAV, causes fulminant hepatic failure more often than HCV infection. However, in the majority of patients, the etiology of fulminant hepatic failure remains unknown.