Ursodeoxycholic acid (UDCA) is a safe and effective medical therapy for most patients with primary biliary cirrhosis (PBC), but some patients show an incomplete response. Silymarin is a potent antioxidant with immunomodulatory and antifibrotic properties. The aim of this study was to evaluate the safety and assess the efficacy of silymarin in patients with PBC who had shown a suboptimal response to UDCA. Twenty-seven patients with PBC who had been on UDCA (13-15 mg/kg/day) therapy for 7 to 221 months and had shown a persistent elevation of alkaline phosphatase activity at least 2 times the upper limit of normal for more than 6 months were enrolled. Oral silymarin, 140 mg 3 times daily was given for 1 year, and patients continued on the same dosage of UDCA. No significant changes in serum alkaline phosphatase activity (897 ± 84 vs. 876 ± 95, P = .5), total bilirubin (0.9 ± 0.1 vs. 1 ± 0.1, P = .07), aspartate transaminase (AST) (58 ± 5 vs. 56 ± 6, P = .4), albumin (4.0 ± .06 vs. 4.1 ± .06, P = .4), or Mayo risk score (3.82 ± 0.2 vs. 3.88 ± 0.2, P = .4) were noted after 1 year of treatment with combination therapy. Transitory gastrointestinal adverse events occurred in 2 patients. In conclusion, although silymarin was well tolerated, this medication did not provide benefit to patients with PBC responding suboptimally to UDCA. The results of this pilot study would seem to discourage further controlled trials of silymarin in patients with PBC.