Background and objectives: Although inflammatory changes are found throughout the airways of patients with chronic bronchitis (CB), the mechanisms of the pathogenesis of CB are still unclear. The aim of this study was to investigate airways inflammation in patients with and without an exacerbation of CB. Granulocyte macrophage-colony stimulating factor (GM-CSF) levels in serum and bronchoalveolar lavage (BAL) fluid were assayed. Materials and methods: 40 CB patients and 10 never smoking, age and sex matched controls were studied. 20 of the CB patients were studied under baseline conditions (B), and 20 during an exacerbation (E) of CB. Bronchoscopy and bronchoalveolar lavage (BAL) with cytological analysis were performed for 7 males from CB(E) group, and 6 males from CB(B) group. The levels of GM-CSF were determined in sera and in BAL supernatants by a solid phase enzyme immunoassay. Results: There was high significant elevation of serum GM-CSF in both CB (E) and CB(B) groups in comparison with the control group (P<0.0001 & P=0.002 respectively). Both serum and BAL GM-CSF levels were elevated in CB(E) group in comparison with CB(B) group (P=0.0001 & P=0.009 respectively). Also there was significant elevation of BAL neutrophils, eosinophils and lymphocytes in CB(E) in comparison with CB(B) (P<0.05, P<0.01 and P<0.01 respectively). There was only significant positive correlation between BAL GM-CSF and BAL neutrophils count in CB(E) group, while no significant results were detected in CB(B) group. Conclusion: During exacerbations of CB there were changes in the cell populations in BAL of patients consistent with a recruitment of polymorphonuclear leucocytes, eosinophils and lymphocytes in the airway lumen. These recruited different inflammatory cells could work together toward the production of airway abnormalities and lung destruction. The locally elevated levels of BAL GM-CSF might be a cause and /or a result suggesting a role for this cytokine in the inflammatory processes of chronic bronchitis. The elevated levels of serum GM-CSF in chronic bronchitis may be due to the interference of other cytokines, microenvironmental factors in bone marrow and/or other factors.