Serum bile acid (SBA) levels have been proposed as a sensitive test of liver function. However, the diagnostic value of SBA in liver diseases varies with the population studied and with the SBA estimation methods. The aims of this study were to determine the diagnostic value of the oral ursodeoxycholic acid (UDCA) tolerance test and to compare the results with those of the conventional liver function tests. An oral tolerance test using a 500 mg dose of UDCA was carried out in 33 healthy subjects, 23 patients with chronic active hepatitis and 52 patients with liver cirrhosis. Total serum bile acids (TSBA) after oral adminiatration of UDCA and the are under the curve (AUC) were significantly (p<0.0001) higher in patients with chronic active hepatitiis and in patients with liver cirrhosis than in healthy subjects. In order to define the diagnostic value of the oral UDCA tolerance test, sensitivity, specificity and predictive value of positive test [PV (+)] of TSBA were calculated and campared with those of conventional liver function tests. The sensitivity of TSBA was not superior to those of conventional liver function tests. The oral UDCA tolerance test was found to be not suitable as a screening test for chronic liver diseases. However, the specificity and PV (+) of TSBA were higher than those of conventional liver function tests. The PV (+) of a test is an accurate indicator of the presence of a disease and gives the odds that an abnormal result indicates its presence. The oral UDCA tolerance test has the high Specificity and PV (+), hence, this test has great value in the identification of a chronic liver disease when an abnor,mal result is found on conventional liver function tests.