Thymomas are neoplasms known to be frequently associated with autoimmune disorders. Oral lichen planus is an immunologically based, chronic inflammatory oral mucosal disease of undetermined cause. We describe a 79-year-old patient with a 6-month history of generalized oral erosive lichen planus in whom a chest roentgenogram led to the discovery of an anterior mediastinal mass consistent with thymoma. Transsternal complete thymoma resection achieved erosive oral lichen planus regression. The clinical correlations between erosive oral lichen planus and thymoma are presented.