While there is evidence that hepatitis C virus (HCV) does not cause fulminant non-A, non-B hepatitis, the causal agent remains unknown. To evaluate the role of hepatitis B virus (HBV) in this disease, we used a two-step polymerase chain reaction (PCR) to amplify the surface and core regions of HBV DNA in serum and liver samples taken prospectively from twenty-six patients (mean age 36 years, range 1 to 64) with acute hepatic failure undergoing liver transplantation. HBV DNA was absent from the serum of all patients before transplantation. Seventeen patients were diagnosed as having non-A, non-B hepatitis because they lacked serological evidence of hepatitis A virus or HBV infection. Liver samples were taken from twelve of these patients, and six samples were positive for HBV DNA. By contrast HBV DNA was not detected in liver from three patients with acute liver failure caused by hepatitis A or toxins. HCV RNA was not found in pretransplant samples by PCR. Four of the six patients with detectable HBV DNA in liver and presumptive non-A, non-B hepatitis had detectable HBV DNA in serum after transplantation. One additional patient who did not donate pretransplant liver had HBV DNA in a post-transplant serum sample. Thus, HBV DNA was present before or after transplantation in seven of seventeen patients with apparent non-A, non-B hepatitis. Three of five patients with detectable post-transplant serum HBV DNA were serologically positive for HBV surface antigen. These findings indicate that HBV may be a common cause of fulminant hepatic failure in patients lacking serological evidence of HBV infection.