BACKGROUND AND AIMS: Quantification of the portosystemic shunt is important for properly managing the patients, predicting prognosis and adjusting dosages of oral iirugs for chronic liver disease. In this study, we evaluate the clinical significance of thallium. 01 liver scan for chronic liver diseases mainly in the aspects of radioactivity distribution. MethndIs: From February 1991 to April 1994, 54 patients with chronic liver disease (25 patients with chi anic active hepatitis, 22 Child A cirrhotics, and seven Child B cirrhotics) underwent the scan. The shunt index (H/L ratio) was determined and was classified into three groups according to the patterns of distributioo of radioactivity performed in them (Type I: normal, Type II: filling defect in the liver with or without extra hepatic radioactivity, Type III: extrahepatic radioactivity only) RESULTS: The shunt index in cirrhotics was higher than chronic active hepatitis (0.56+/-0.38 vs 0.35+/-024 p<0.05). In terms of patterns of distribution of radioactivity l3 patients among 14 patients with type I (93%) were patients with chronic active hepatitis, 14 patients among 25 patients with type II (56%) were cirrhotics and the remaining were patients with chronic active hepatitis. 14 patients among 15 patients with type Ill were cirrhotics. In terms of shunt indices according to the patterns of distribution of radioactivity the shunt index of type III was higher than that of type II and that of type II was higher than type I (0.79+/-0,40:0.39+/-0.29 p<0.01, 0.39+/-0.29:0.21+/-0.14 p<0.05). CONCLUSIONS: Determination of the shunt index using a per rectal thalliurn-201 liver scan wa:> a useful method for quantifying the porto-systemic shunt noninvasively and it can differentiate chroruc active hepatitis from cirrhosis. Considering that the graver the chronic liver disease is, that moie frequent type III is, we assume that transformation from type I to type III indicates a progres. Ion of chronic active hepatitis to cirrhosis.