Nitric oxide (NO) has a complex role in the development of skin disorder. NO is synthesized by the enzyme nitric oxide synthase (NOS), in which three NOS isoforms: NOS1, NOS2 and NOS3 have been identified. A previous study had shown that little or no NOS2 expression was found in OLP. However, the correlation between OLP and three types of NOS has not been thoroughly studied. Objective: The aim of this study was to determine whether the expression of three types of NOS was up-regulated in OLP relative to non-inflamed oral fibroma (OF). Methods: Seventeen OLP and seven OF biopsies were studied by using immunohistochemical technique to detect all three types of NOS. Results: It was found that both NOS2 and NOS3 were expressed in the epithelial layers of OLP and OF conditions, while there was no NOS1 expression in the epithelium from both conditions. However, NOS1 was positively expressed in the neurons. NOS2 (65%) and NOS3 (94%) expression in OLP condition were greater than those in OF condition (57%). The pattern of NOS2 and NOS3 expression between OLP and OF were different. The pattern of NOS2 and NOS3 expression were in the suprabasal and basal cells in OLP condition, but NOS2 and NOS3 positive cells in the OF condition were found only in the suprabasal cells. Conclusions: These findings suggest that the NOS2 and NOS3 were only expressed in the basal cells of the inflammatory OLP lesions. Therefore, NOS and their product, NO, may play an important role in the immunopathogenesis of OLP. (Supported by MRG4680144 and intramural funding of Faculty of Dentistry, Chiang Mai University to SC and TRF4580028 to SK.)