We and others have shown a normalizing effect of hormone replacement therapy (HRT), containing 17bestradiol and a gestagen, on liver enzymes, which may point towards a protective effect on hepatocyte integrity. Marked architectural changes, including nodular regenerative hyperplasia, multiple focal nodular hyperplasia and cirrhosis are observed in some patients and are associated with a risk of liver-related complications. These changes are frequently associated with vascular disorders such as obliterative portal venopathy, probably related to congenitally abnormal vessels. Steatosis, steatofibrosis, and steatohepatitis are seen and may be caused by metabolic disorders. In addition, bile duct alterations resembling small duct sclerosing cholangitis are observed in several patients. Presently, it is not known whether these perturbations in liver morphology and in liver-derived enzymes are related to functional defects in females with TS and whether this may change by HRT. To further explore quantitative liver function in TS, we examined adult women with TS on and off HRT and compared them with a control group of age matched normal women. We used the galactose elimination capacity to assess hepatocyte cytosol activity, the plasma clearance of indocyanine green to assess hepatic blood flow and excretory liver cell function independently of hepatic blood flow, the antipyrine plasma clearance to estimate hepatic microsomal system activity, and the functional hepatic nitrogen clearance to assess mitochondrial- cytosolic metabolic capacity for conversion of amino-nitrogen.We assumed that one or more these metabolic liver functions would be diminished in untreated TS and normalized by HRT. Our principal objective was to understand mechanistically how HRT improves liver function in TS.