Lichen planus (LP) is the most frequent dermatologic disorder presenting oral involvement. It is estimated that LP affects 1% of the general population. The causative agent as well as the etiology are unknown. Recent studies tend to implicate the role of T4 and T8 lymphocytes as well as the Langerhans cells and the epithelial antigenic macrophage. Recently it has been suggested that the immune system may play a critical role in the development of oral LP in patients with type I diabetes mellitus. Also there is evidence suggesting a significant association between hepatitis C viral infection and LP in some groups of patients. The mechanism of this association is not known but possibly involves cell mediated response to an altered epithelial antigen.

Other etiological co-factors mentioned for LP are emotional stress, genetic predisposition, hypersensitivity to drugs, general debilitation and bacterial or viral infections. Presently it is considered as a disease of multifactorial etiology.

The development of a squamous cell carcinoma in a pre-existing oral LP has been postulated and debated for many years, presently this association is estimated to occur, mostly on the erosive type of LP, in 0.4% to 2% of cases of LP especially when LP is present for more than 5 years. Squamous cell carcinomas as well as epithelial dysplasia are microscopically characterized by an abundant lymphocytic infiltrate in the connective tissue subjacent to the epithelial changes. This microscopic appearance is similar to that seen in LP, hence, many cases of either early squamous cell carcinoma and the so-called lichenoid dysplasia can be mistaken by an early malignancy or pre-malignancy developing on an LP.

Additionally there are several exogenous agents able to cause oral mucosal lesions which present a histological image similar to that of LP. Those lesions are grouped under the name of lichenoid reactions or lichenoid lesions. These agents induce the mucosal changes by contact, among the most frequently cited are, amalgam (mostly due to mercury), gold and methacrylic acid esters.

Patients undergoing bone marrow transplants or massive transfusions may develop a condition known as graft-versus-host disease. Part of the host reaction is the production of skin and mucosal lesions which are histologically identical to LP. Lesions in these patients may precede the development of more severe manifestations which can lead to the demise of the patient.