Dynamic concept of oral lichen planus. The diagnosis easy at early stages may become difficult in ancient lichen planus. Lichen planus is a chronic inflammatory dermatosis of the skin, skin appendages and mucous membranes, which frequently affects the oral mucosa. Its aetiology still remains unknown, and currently accepted pathogenesis is that of an autoimmune cell-mediated disease. To the contrary of skin lichen planus, oral lichen planus is a long-term chronic disease with dynamic evolution, in which progressive and profound changes of the clinical and histopathological aspects occur over time and under the influence of various exogenous factors. By convention, in the history of the oral lichen planus four successive stages can be distinguished without well-defined boundaries between them. These stages can be defined as an initial phase; a long intermediate phase with alternating periods of activity and quiescence, which has a gradually increasing risk of malignant transformation; a late stage which activity is traditionally diminished; and a post-lichen cicatricial stage with an absent or negligible and undetectable activity, often undiagnosed because clinically unrecognized; in this stage, the lesion does not respond to usual treatments, but retains the same risk of malignant transformation.