Abstract: Objective To investigate the levels of complement and activated co mplement factors in peripheral blood and skin in patients with psoriasis and the ir significance in the development of psoriatic lesion. Methods Enzyme linked im munosorbent assay (ELISA) and immuno.turbidi metric analysis were used to dete rmine the levels of C3d, sC5b-9, C3 and C4 in sera from 57 patients with psori asis vulgaris. Immunohistochemistry was employed to assess in situ expression of SC5b-9 in lesional and non.lesional skin of 37 patients. Results There was a significant decrease of C3 and C4, and increase of C3d and sC5b-9 in patient s with psoriasis in comparison with those of normal controls (t=10.88～24.80, P<0.01). The levels of C3 and C4 were significantly lower, while the levels of C 3d and sC5b-9 were significantly higher in progressive stage than those in sta tic stage (t=4.55～6.77,P<0.01). In situ expression of sC5b-9 was significan tly higher in stratum corneum and dermo.epidermal junction of lesional skin th an that in non.lesional skin and normal controls (χ²=62.90,45.45;P<0.01). T here was no significant difference of C5b-9 expression in stratum corneum and dermo.epidermal junction of lesional skin between progressive and static stage. Conclusions There is significant correlation between the development of psoria tic lesion and local deposit of activated complement factors.

Key words: Psoriasis, Complement