Objectives: A prospective study was performed to find out the correlation between neutrophil, eosinophil and eosinophilic cationic protein (ECP) the granular protein in eosinophil - and smoking history and pulmonary function change, Methods: In this study there were seventy smokers and seventy lifetime nonsmokers. None of them had the history of atopy, or the evidence of parasitic infestation. Smoking history, spirometric pulmonary function test and blood samples for CBC, serum ECP were analyzed. Results: 1) Eosinophil and neutophil count was elevated in the smokers compared with the nonsmokers. 2) Serum ECP was elevated in smokers and the magnitude of this increase was greater than that of eosinophil count. 3) In the smokers, there was a direct proportional correlation between serum ECP and FEV1, but inverse proportional correlation between neutophil count and FEV. 4) In the smokers, neutrophil count was elevated regardless of FEV value. 5) The level of serum ECP well correlated with the daily tobacco consumption but not with the duration of smoking or pack-year consumption. Conclusion: Incresed neutrophil count, decreased eosinophil count and decreased serum ECP level could be useful in the understanding of pathophysiology of lung parenchymal destruction in smokers.