Peripheral blood dendritic cells seem to play a crucial role in psoriatic inflammatory processes. The aim of our study is to investigate the relationship between plasma interleukin-18 (IL-18) levels and blood dendritic cells in psoriatic patients. IL-18 plasma levels were measured by ELISA. Phenotypes of dendritic cell subsets were analyzed by double-colour flow cytometry. Plasma IL-18 level in psoriatic males was significantly higher, whereas counts of BDCA-2+ cells were lower than in the control group. The myeloid/plasmacytoid ratio was significantly higher in the patient group compared to the control one. In the patient group, significant negative correlations between plasma IL-18 level and both the BDCA-1+ and BDCA-2+ counts were found. BDCA-1+ counts correlated negatively with percentage of skin involvement. IL-18 seems to play a role in psoriasis pathogenesis. The decreased counts of blood plasmacytoid DCs in psoriatic patients might result from IL-18 down-regulation of plasmacytoid DC precursor proliferation.