To examine bile acid synthesis in chronic liver diseases, serum total 7a-hydroxycholesterol level was measured by gas-liquid chromatography-mass spectrometry in patients with cirrhosis (n = 23), patients with chronic hepatitis (n = 21), and control subjects (n = 18). The serum 7a-hydroxycholesterol levels were significantly lower in patients with cirrhosis than the controls (78 2 59 pmoVmL vs. 237 & 97 pmoVmL; mean & SD). However, in patients with chronic hepatitis, the level was fully retained (262 2 102 pmoVmL). Serum 7a-hydroxycholesterol levels of 17 patients with cirrhosis classified as Child B and C ranged from 33 to 69 pmoVmL, and all were less than the normal range (between 104 and 466 pmoYmL), however, those levels of some patients classified as Child A were within the normal range. Serum 7a-hydroxycholestero1 levels significantly correlated with serum albumin, cholinesterase, total bile acid, direct bilirubin, alkaline phosphatase, indocyanine green (ICG) retention rate, hepaplastin test, and lecithin-cholesterol acyltransferase activities. We conclude that bile acid synthesis is well preserved in patients with chronic hepatitis and that it is decreased in most patients with cirrhosis. Serum 7a-hydroxycholesterol may be a new parameter of liver function testing to assess hepatic bile acid synthesis in patients with chronic liver diseases. (HEPATOLOGY 1995;22:1182-1187.) Bile acid synthesis is one of the most important functions of the liver and its rate-limiting step is catalyzed by hepatic microsomal cholesterol 7a-hydroxylase (EC 1.14.13.17).l In humans, bile acid synthesis has been evaluated by some cumbersome methods, such as measurement of fecal bile acid,' kinetic studies with radiolabeled or stable i s o t o p e ~, ~, ~ and determination of cho-Abbreviations: AST, aspartate transaminase; ALT, alanine transaminase; ALP, alkaline phosphatase; 7-GTP, y-glutamyltranspeptidase; LCAT, lecithincholesterol acyltransferase; HDL, high-density lipoprotein; ICG, indocyanine green.