Ursodeoxycholic acid (UDCA) has beneficial effects in cholestatic liver diseases. In primary sclerosing cholangitis (PSC), there is evidence that high doses (±20 mg/kg) of UDCA may be more effective than average doses. Biliary enrichment of UDCA at such high doses may represent the decisive factor for its beneficial effect. Up to now it is not clear how high-dose UDCA correlates with its biliary enrichment and whether bacterial degradation of large amounts of UDCA may lead to an increased bacterial formation of more toxic hydrophobic bile acids. We determined the biliary bile acid composition in 56 patients with PSC including 30 patients with repeat bile samples treated with various doses of UDCA. At a UDCA dose of 10–13 mg/kg/d (n = 18) biliary UDCA represented 43.1% + 0.3% (mean + SD) of total bile acids; at a UDCA dose of 14–17 mg/kg (n = 14), its biliary content increased to 46.9% + 0.3%, at 18–21 mg/kg (n = 34) to 55.9% + 0.2%, at 22–25 mg/kg (n = 12) to 58.6% + 2.3%, and at 26–32 mg/kg (n = 8) to 57.7% + 0.4%. During UDCA treatment, the biliary content of all other bile acids was unchanged or decreased. In conclusion, biliary enrichment of UDCA increases with increasing dose and reaches a plateau at 22–25 mg/kg. There was no increase of toxic hydrophobic bile acids. If biliary enrichment of UDCA represents the decisive factor for its clinical effect, it seems likely that UDCA doses of up to 22–25 mg/kg may be more effective than lower doses. (HEPATOLOGY 2004;40:693–698.)