Abstract : The serum level of IL-17 was evaluated in patients with oral lichen planus (OLP) treated by topical corticosteroids. Ten patients diagnosed with erosive/atrophic OLP were included in this study. The patients were instructed to apply kenacort -A orabase pomad 4 times daily for 1 month. Serum samples were collected at baseline, immediately after first session, 2 months & 4 months (end of the study) from baseline for determination of serum level of IL-17 using enzyme-linked immunosorbent assay (ELISA). The mean level of IL-17 was high in serum at baseline, after topical application of corticosteroids the IL-17 level was decreased from 579.9±202.82 to 400.4±163.43, the p-value was < 0.001 and the results were statistically significant. According to our results, IL-17 plays a main role in the pathogenesis of OLP and its serum level may be valuable in the diagnosis of erosive/atrophic OLP. Topical corticosteroids can be used as an effective method of treatment in OLP.