We determined whether the normalization of serum bilirubin level (SBL) induced by ursodeoxycholic acid (UDCA) therapy was associated with an improved clinical outcome in patients with primary biliary cirrhosis (PBC). We estimated the prognostic values of SBL measured after 6 months of UDCA treatment for survival free of orthotopic liver transplantation (OLT). We used a database of 548 patients with PBC followed in three trials of UDCA. Among UDCA-treated patients, we compared survival free of OLT in patients with normalized SBL (</=17 micromol/L) with those who had persistently elevated SBL. Difference in survival was tested between UDCA-treated patients whose SBL normalized with treatment and placebo patients who had normal baseline SBL. We evaluated, in each treatment group, the prognostic value of 6-month SBL. Survival was estimated using the Kaplan-Meier method and compared by the Cox model. Survival free of OLT was significantly longer in patients who had normalized SBL (P <. 0001; relative risk [RR]: 3.7, UDCA group). Survival free of OLT was not significantly different between UDCA patients with normalized SBL and placebo patients with a normal baseline SBL (P =.69). For several cutoffs of 6-month SBL, RRs of OLT or death were similar in UDCA-treated and placebo patients: the RR of OLT or death associated with a 6-month SBL more than 30 micromol/L was 6.0 for UDCA and 5.7 for placebo groups. In conclusion, normalization of SBL during therapy is associated with improved clinical outcome. SBL under UDCA therapy is a prognostic factor in PBC. SBL under UDCA therapy should be interpreted as in untreated patients.