Background: Asthma is the chronic inflammation of airways characterized by eosinophilic infiltration, mucus overproduction, airway hyper-responsiveness and airway remodeling. These changes are induced mostly by cytokines which are produced by T helper (Th) 2 cells. Recently, the role of interleukin-23 (IL-23) in the pathogenesis of adultallergic asthma has been studied. Objective: To explore IL-23 serum levels and its expression in persistent asthma compared with healthy children younger than five years old. Method: Blood samples of 40 children with mild and severe persistent asthma were compared to 34 healthy children regarding IL-23 serum levels and gene expression using enzyme-linked immunosorbentassay (ELISA) and real time quantitative polymerase chain reaction (PCR). Results: The IL-23 gene expression level was significantly different in the 25 children with mild persistent asthma and the 15 children with severe persistent asthma compared to the control group (p=0.001).There was no significant difference in IL-23 gene expression level between the two groups of patients with mild and severe persistent asthma. A significant difference was seen in IL-23 serum levels between the 25 children with persistent asthma and control group (p=0.002).Conclusion: For pre-school children with history and physical exam in favor of asthma which cannot be tested by spirometry, IL-23 serum levels may be an auxiliary biomarker for the diagnosis of asthma.