Objective: To assess the distribution pattern of Langerhans cells in oral lichen planus using adenosine triphosphatase (ATPase) histochemistry and to correlate this with the duration of symptoms. Method and Materials: Included were fresh unfixed tissues from previously untreated cases that were clinically and histologically diagnosed as oral lichen planus (n = 18). Healthy oral mucosal tissues were used as controls (n = 5). ATPase activity in the Langerhans cells from the tissue samples was assessed by enzyme histochemistry. Results: The difference in the distribution pattern of Langerhans cells in the superficial and basal half of the epithelium in both the lesional and control tissue was significant (P .001, Mann-Whitney U test). With an increase in duration of symptoms, the median migration of cells from superficial position to the basal half was significant (P = .001, Kruskal-Wallis test). A negative correlation (Pearson correlation coefficient) was seen between duration of symptoms and migration of cells. Conclusion: The initial increase in the number of Langerhans cells in the lesional tissues compared to controls suggests that these cells are critically required for both the initiation and progression of the mucosal immune response in oral lichen planus. Variation in their number and distribution bear clinical importance as objective assessment of long-standing lesions could be made with reference to treatment planning. Palabras clave: ATPase, enzyme histochemistry, immunopathogenesis, Langerhans cells, oral lichen planus, special stains