We used a specific method to measure conjugated bilirubin levels in patients with acute liver diseases to examine its clinical usefulness. Conjugated and total bilirubin levels were measured in 102 samples obtained from six patients with acute liver diseases (three with fulminant hepatic failure, one with acute severe hepatitis and two with acute hepatitis; see text for criteria). Total and conjugated bilirubin levels were measured with Iatro T (total)-Bil and D (direct)-Bil kits (Iatron Laboratories Tokyo, Japan) and with conventional Nescauto T(total)-Bil and D(direct)-Bil VE kits (Nippor Shoji, Osaka, Japan). The Iatro D-Bil kit measures conjugated bilirubin correctly, while the Nescauto D-Bil VE kit measures some nonconjugated bilirubin and delta bilirubin as well as conjugated bilirubin. Total bilirubin levels determined by the two methods showed good correlation. The conjugated bilirubin level measured with the Iatro D-Bil kit was strongly correlated with the direct bilirubin level measured with the Nescauto D-Bil VE kit, but there was no correlation between the conjugated-to-total bilirubin ratio and the direct-to-total bilirubin ratio. When we examined the changes in bilirubin levels in our patients with respect to outcome, we found that the two patients in whom the ratio of conjugated-to-total bilirubin exceeded 0.3 died, while all four patients in whom the ratio remained below 0.3 survived. The ratio of direct-to-total bilirubin was unrelated to outcome. The conjugated bilirubin level measured with the Iatro kits was therefore considered useful for the diagnosis and follow-up of acute liver diseases.