Objective To assess the number of peripheral blood Th17 cells and mRNA expressions of IL-17A and IL-23R and their correlations with disease severity in patients with psoriasis vulgaris (PV).Methods Tissue specimens were resected from the lesions of 25 patients with PV and normal skin of 10 human controls,and venous blood samples were obtained from 20 of the patients and all of the normal human controls.Reverse transcription PCR and flow cytometry were carried out to measure the mRNA levels of IL-17A and IL-23R in these tissue specimens and quantify the number of peripheral blood CD4+IL-17+ T lymphocytes.Psoriasis area and severity index (PASI) was calculated for these patients.Results A significant increase was observed in the IL-17A and IL-23R mRNA levels in the patients compared with the controls (0.996 ± 0.231vs.0.437 ± 0.096,t =10.572,P ＜ 0.05; 1.006 ± 0.339 vs.0.491 ± 0.196,t =6.015,P ＜ 0.05).The levels of both IL-17A mRNA and IL-23R mRNA were positively correlated with PASI (r,=0.67,0.70,respectively,both P ＜ 0.05).The number of CD4+IL-1 7+ T lymphocytes in peripheral blood showed no significant differences between the patients and controls.No statistical correlation was observed between the counts of CD4+IL-17+ T lymphocytes and expression levels of IL-17A or IL-23R mRNA in psoriatic lesions.Conclusions In patients with PV,there is an increase in the expressions of IL-17A and IL-23R mRNA in lesions,which are correlated with disease severity,while no significant change is observed in the number of peripheral blood CD4+IL-17+ T cells.