Background: Interleukin-33 is a member of IL-1 cytokine family that acts as an important mediator to inflammation in asthma and other allergic diseases. Objective: To study the relation of serum IL-33 level to disease severity in asthmatic patients. Methodology: Serum IL-33 and IgE levels in 40 asthmatic patients and 20 control subjects were measured by ELISA. Patients were classified as having intermittent, mild, moderate or severe asthma. The relation of serum IL-33 to disease severity, serum IgE, presence of atopy and other allergic diseases was evaluated. Results: IL-33 and IgE levels were higher in asthmatic patients compared to controls. These levels showed significant difference in different degrees of disease severity and significant increase in atopic asthmatics compared to non-atopic patients. No difference was found between patients with asthma only and asthmatics with other allergic diseases. Conclusion: Serum IL-33 is in direct relation to disease severity in asthmatic patients. It is higher in atopic asthmatics than in non-atopic patients.