Hashimoto's thyroiditis is the most frequent autoimmune disease with genetic and environmental aetiologies. Viral infections have been postulated as one of the factors that may trigger autoimmune diseases. Many studies suggest that Herpes simplex virus infections are involved in a variety of autoimmune diseases. We report the case of three patients presenting for the first time herpes labialis a few months before the onset of hashitoxicosis. Serological and clinical exams support the possible role of human herpes viruses in the aetiology of Hashimoto's thyroiditis.