In the correlation between IL-18 expression and different demographic and behavioural characteristics of the studied patients as one group, a significant correlation was obtained between IL-18 expression levels in psoriatic lesions and the severity of psoriasis expressed by PASI score (r = 0.72 and P = 0.001) and disease duration (r = 0.40 and P = 0.01). No significant correlation was revealed between IL-18 expression levels in psoriatic lesions and patients' age, smoking status, predisposing factors or coexisting diseases.