To evaluate thyroid function in 19 patients with fulminant hepatitis (FH), we have measured total and free 3,5,3′-triiodothyronine (T3) and thyroxine (T4), 3,3′,5′-triiodothyronine (reverse T3, rT3), thyroidstimulating hormone (TSH) and thyroxin-binding globulin (TBG) in patients with FH, compared with those of 80 patients with other various liver diseases and of 10 healthy controls. Patients with FH showed the lowest values of serum T3 and the highest levels of rT3 among all patients with liver diseases studied. Furthermore, patients with FH showed a significant increase of rT3 in comparison with subacute hepatitis (SAH), “acute-on-chronic” (AOC) type of hepatic failure, ordinary and severe forms of acute hepatitis (AHo and AHs) and decompensated liver cirrhosis (LC-D). In addition, serum T3 and rT3 and the rT3/T3 ratio significantly correlated with prothrombin time (PT) and plasma methionine level. We also found that serum T3 and rT3 concentrations and the rT3/T3 ratio showed early and rapid normalization in cases of FH that survived, but they did not improve in patients with fatal outcome. These results suggest that serum T3, particularly rT3 concentrations and the rT3/T3 ratio may be useful indicators for assessing the severity and prognosis of patients with FH and can be considered to the sensitive indices for functioning hepatic microsomal reserve as well.