Background: psoriasis is a chronic inflammatory autoimmune skin disorder, with a high relapse rate. The etiology and pathogenesis of psoriasis are still unclear. Many cytokines produced by inflammatory cells give rise to the induction and maintenance of psoriatic plaques. YKL-40 or chitinase 3-like protein1(CHI3L1) may be involved in angiogenesis in psoriasis which has an important role in the pathogenesis of the disease. Objectives: The aim of this study was to study the serum level of YKL-40 in patient with psoriasis in order to assess its possible role in pathogenesis and severity of the disease. Patients and method: this study included 60 psoriatic patients and 30 healthy subjects of matched age and sex, served as a control group. Blood samples were taken from all patients and controls for estimation of serum level of YKL-40 by ELISA. Results: A statistically significant increase in median serum YKL-40 level in psoriatic patients compared with control group. A positive correlation between serum YKL-40 and severity of psoriasis according to PASI score. Conclusion: serum YKL-40 can be used as a new marker for evaluation of disease severity, progression and therapeutic decision in psoriasis.