The purpose of this graduate thesis is to study the etiology, clinical features, symptoms, diagnosis and therapy of oral lichenoid reaction through a review of literature and a case report. Clinically and histopathologically it resembles oral lichen planus. The main differences to oral lichen are known etiological factors and regression of the disease after their removal. A review of literature on the subject confirmed that the most common factors are systemic medications, amalgam restorations and graft versus host disease. Less common factors include composite resins, glassionomer cement, tobacco and bethel chewing, etc. In addition to mercury, copper, zinc and tin are related to lichenoid reactions as well. Clinical symptoms includ white striae, papulae, plaque, erosions, and atrophy of the oral mucosa. They are not a characteristic appearance of this disease. Accompanying symptoms such as pain, sores and burning sensation caused by hot or spicy food do not make diagnostics any easy. Therefore the diagnostics itself is based on a carefully assembled medical history, especially concerning drugs and must include a detailed inspection of the oral cavity and existing amalgam restorations. Lichenoid reactions are product of old, corroded or unpolished restorations, black surfaces which may release the products of corrosions or metal ions and therefore have a toxic and irritating effect on the mucosa. Treatment of oral lichenoid reaction depends on the etiological factor above all. In case the cause is an amalgam restoration, it is necessary to replace it with composite or ceramic restoration. Lichenoid reactions caused with drugs must be treated by replacing the same drugs with another. In case that is not possible because replacing the same might lead to a life-threatening situation, the reaction should be treated at the same way as oral lichen planus. The thesis also delineates the clinical case of a patient with oral lichenoid reaction caused by an amalgam restoration.