This paper presents the case of a 58-year-old heavy smoker female who came to our clinic with acute pain, as well as mastication and feeding difficulties. The macroscopic examination revealed oral erosive lesions and ulcerations. The polymorphic aspect of the lesions required the differential diagnosis of oral erythroplakia or carcinoma, which were excluded by biopsy. At the same time, we assessed the expression of S100 protein, Ki67 and the cluster of differentiation (CD) 4, CD8 (T-cell) and CD20 (B-cell) immune cell markers by immunohistochemical analysis. As a result, after the clinical and pathological assessment, the diagnosis of oral lichen planus was established, and a therapy plan was conducted. We observed a favorable clinical evolution after the administration of corticosteroids and immunomodulatory agents.