Interleukin-8 (IL-8) is considered as the major polymorphonuclear neutrophils (PMNs) chemoattractant cytokine in lung diseases such as asthma and adult respiratory distress syndrome (ARDS). However, controversial results were obtained regarding the involvement of IL-8 in the pathogenesis of pneumonia. This study examines the role of IL-8 in the recruitment and activation of PMNs in the lung of pneumonia patients. The interesting aspect of this study is that it is a site- specific analysis of the infected and uninfected lungs of the same patient. The level of IL-8 mRNA, protein and myeloperoxidase present in the cells of the bronchioalveolar lavages (BALs) taken from the areas of known pneumonic consolidations on chest X-ray (infected lung) are compared with the BALs obtained from areas of no obvious infiltrate (non-infected lung). The results obtained from the infected and non-infected lungs of pneumonic patients were further compared with that of a control group of non-smoking patients. The level of IL-8 mRNA and protein were determined by RT-PCR and ELISA respectively. There was a significant increase in the level of IL-8 mRNA in the infected lung as compared to its level in the non-infected lung (p < 0.001). In correlation with the increase in mRNA, IL-8 protein concentrations in BAL fluids from the infected lung were 6 fold higher than those taken from the non-infected lung (p < 0.0001). This pattern was also consistent with MPO activity in the BALs (4.5 fold more MPO activity in the infected lung as compared to that of the non-infected lung), indicating that IL-8 is directly implicated in neutrophil accumulation that follows acute respiratory infection. The results of the present study, therefore, indicate the involvement of IL-8 in the pathogenesis of pneumonia.