Oral lichen planus (OLP), an intractable inflammatory disease characterized by a band-like lymphocytic invasion under the oral mucosa, is frequently associated with hepatitis C virus (HCV) infection. We investigated the effects of glycyrrhizin, which is used to treat chronic liver dysfunction, in nine patients with OLP who were positive for HCV antibody and HCV RNA. A control group, eight patients with OLP who were also positive for HCV antibody and HCV RNA, was given only dental cleaning. Glycyrrhizin (GL) was given intravenously, at a dose of 40 ml (0.2% solution) daily, for 4 consecutive weeks. Six (66.7%) of the nine patients given GL improved clinically (P = 0.0141 vs non-GL group), suggesting that GL is useful in treating OLP.