From September 9 to October 3, 1994, five patients on maintenance hemodialysis in a dialysis unit in Tokyo contracted hepatitis B virus (HBV) infection successively, and four of them died of fulminant hepatitis. The unit treated 181 patients three times a week on eight shifts, and all five afflicted patients were on the same shift along with 27 other patients. HBV DNA clones from the hepatitis patients had a point mutation converting codon 28 in the precore region to a stop codon, which aborts the synthesis and secretion of hepatitis B e antigen, and showed a sequence similarity of > 99.5% within 645 base pairs covering the X gene and precore region. There were two HBV carriers with antibody to hepatitis B e antigen who were receiving hemodialysis on the same shift. HBV DNA clones from one of them had the stop codon 28 in the precore region, and a sequence similarity of > 99.7% to those from the five patients. Based on these results, it was deduced that the fulminant HBV strain was transmitted from the carrier to five patients, and resulted in the death of four. The outbreak indicates that immunocompromised hosts like hemodialysis patients can develop fulminant hepatitis B if and when they are infected with extremely virulent HBV strains.