Herbal products, used for centuries in Far Eastern countries, are gaining popularity in western countries. Surveys indicate that persons with chronic hepatitis C (CHC) often use herbals, especially silymarin (milk thistle extract), hoping to improve the modest response to antiviral therapy and reduce side effects. The Hepatitis C Antiviral Long-Term Treatment Against Cirrhosis (HALT-C) Trial, involving persons with advanced CHC, nonresponders to prior antiviral therapy but still willing to participate in long-term pegylated interferon treatment, offered the opportunity to examine the use and potential effects of silymarin. Among 1145 study participants, 56% had never taken herbals, 21% admitted past use, and 23% were using them at enrollment. Silymarin constituted 72% of 60 herbals used at enrollment. Among all participants, 67% had never used silymarin, 16% used it in the past, and 17% used it at baseline. Silymarin use varied widely among the 10 participating study centers; men were more frequent users than women, as were non-Hispanic whites than African Americans and Hispanics. Silymarin use correlated strongly with higher education. No beneficial effect of silymarin was found on serum alanine aminotransferase or hepatitis C virus (HCV) RNA levels. Univariate analysis showed significantly fewer liver-related symptoms and better quality-of-life parameters in users than nonusers, but after reanalysis adjusted for covariates of age, race, education, alcohol consumption, exercise, body mass index, and smoking, only fatigue, nausea, liver pain, anorexia, muscle and joint pain, and general health remained significantly better in silymarin users. In conclusion, silymarin users had similar alanine aminotransferase and HCV levels to those of nonusers but fewer symptoms and somewhat better quality-of-life indices. Because its use among these HALT-C participants was self-motivated and uncontrolled, however, only a well-designed prospective study can determine whether silymarin provides benefit to persons with chronic hepatitis C.