Abstract： Objective To investigate the internal relationship between the expression of inducible nitric oxide synthase (iNOS) and the activation of NF-κ Bp65 in the skin of patients with psoriasis vulgaris(PV) and their roles in the pathogenesis of PV.Methods The expression and distribution of iNOS and NF-κ Bp65 were studied by in situ hybridization and immunohistochemistry in 20 patients with PV and 12 healthy controls.The severity of PV was assessed by Psoriasis and Severity Index(PASI) score.Results ① The expression of both iNOS mRNA and protein was significantly higher in lesional epidermis than those in non-lesional epidermis and epidermis of the controls (P<0.01).Weak exprssion of iNOS mRNA was found in basal cell layer in a small number of healthy controls, however, strong expression was observed in entire basal cell layer and focal parts of prickle cell layer in all PV epidermis.② There was significantly positive correlation between iNOS and NF-κ Bp65 protein expression in lesional epidermis (P< 0.01).③ There was significantly positive correlation between the level of expression of iNOS mRNA in lesional epidermis and PASI score of patients (P< 0.01).Conclusions The expression of iNOS and NF-κ Bp65 is markedly increased in lesional epidermis of patient with PV.The activation of NF-κ B probably takes part in the pathogenesis of PV.High-level expression of iNOS may be due to the activation of NF-κ B.