Introduction: The classical Th1/Th2 paradigm previously defining atopic dermatitis, psoriasis and lichen planus has recently been challenged with the discovery of Th17 that is now recognized to produce a group of distinctive cytokines such as interleukin 17 under the control of interleukin 23. Aim: to evaluate the role of Interleukin 23 and interleukin17 in the pathogenesis of psoriasis, atopic dermatitis and lichen planus. Methods: The study included three groups of patients, psoriasis, atopic dermatitis and cutaneous lichen planus each group containing 20 patients, in addition to 20 age and sex matched healthy controls. The levels of IL23 and IL17A were determined in serum samples by ELISA and correlated with the disease severity which was evaluated using specific severity index for each disease. Results: The serum levels of IL23 and IL17 were found to be significantly higher in the diseased groups when compared to healthy controls. The highest levels of IL-23 and IL-17 were reported in psoriasis vulgaris followed by lichen planus and the lowest levels of IL23 and IL17 were reported in atopic dermatitis. The correlation of IL23 and IL17 levels with disease severity was not statistically significant in the three groups. Conclusion: IL23 and IL17 may play a role in the pathogenesis of the three diseases but cannot be used as a marker of disease severity. On the other hand, the significant positive correlation between IL-23 and IL-17 in the three diseases suggests that those factors affect IL-23 will be reflected on IL-17 levels.