Abstract OBJECTIVES: Hashimoto's thyroiditis as well as lichen planus has been associated to a number of disorders, generally of auto-immune origin. A novel possible association between oral lichen planus (OLP) and Hashimoto's thyroiditis (HT) is here proposed on the basis of a cross-sectional survey. MATERIALS AND METHODS: One hundred and five unrelated OLP patients were considered. Diagnosis of HT was based on positive serum anti-TPO, anti-Tg, TSH levels and the typical ultrasound pattern of the thyroid gland. RESULTS: In the present survey, the prevalence of HT in the OLP group was 14.3 % whereas the prevalence of HT-related hypothyroidism in the general population was reported to be equal to 1 %. By Fisher's exact test, it was revealed that the difference between our data and historical prevalence of HT was found statistically significant. CONCLUSION: Actually, there is no definitive hypothesis that could explain the coexistence of OLP and HT. However, considering the onset timing of HT followed by OLP in 93.3 % of our series, we suspected a causal or predisposing role for HT. Specifically, we believe that in HT patients, circulating thyroid antibodies could contribute to trigger an organ-specific auto-immune response also in the oral mucosa or skin, leading to the development of LP lesions. CLINICAL RELEVANCE: Because of the large number of cases of asymptomatic chronic auto-immune thyroiditis, it would be useful that women over 40 years of age affected by OLP were screened for thyroid dysfunction, particularly HT.