Myeloperoxidase (MPO) is mostly released from activated neutrophils during inflammation and act locally in the airways and other pulmonary compartments.The aim of this study was to evaluate serum MPO as an inflammatory marker in inflammatory lung diseases and correlation of MPO to the severity of lung diseases.Methods: This cross sectional study was carried out on 96 children ;selected as32patients with CF at exacerbation,32 with acute pneumonia and 32 as negative control group. Careful history taking, detailed clinical examination and specific investigations including complete blood count (CBC) and serum level of MPO.Results: MPO was significantly higher in patients with CF and pneumonia when compared to healthy control group,(t= 7.8, p value <0.001) . Correlation of MPO level to the degrees of respiratory distress revealed a high statistical significance (rho 0.365, P value 0.003).Correlation of MPO level to the results of TLC revealed a high statistical significance (t= 7.1, P value <0.001) .The result of comparison of MPO level between pneumonia and CF cases revealed a high statisticalsignificance (t= 3.9, P value <0.001 ) .In conclusion: MPO has been identified as a inflammatory marker and its level was correlated with the severity of inflammatory lung diseases(CF and pneumonia) through correlation of its level to the degree of respiratory distress and result of TLC.