Allergy for mercury in dental alloys can be responsible for the induction of allergic contact stomatitis. Evidence is also available for a role of mercury sensitization in the pathogenicity of oral lichen planus in particular. Aspects related to the development of allergic processes in the oral cavity are summarized. Allergy for mercury as the cause of oral mucosal reactions is rather rare in spite of the frequent use in dental amalgam, but probably underestimation of the incidence should be considered. Recent views with respect to the role of mercury as allergen in allergic contact stomatitis, oral lichen planus, the so-called Burning mouth syndrome and in systemic allergic reactions are summarized. Immunologic aspects in the pathogenesis are briefly discussed. Especially in cases in which the site or oral lesions is opposite to the dental alloy it is important to establish sensitization for mercury by use of epicutaneous path testing. When a positive test to mercury is obtained replacement of mercury containing alloys should be considered.