Airway remodeling in asthma and chronic obstructive pulmonary disease (COPD) results in thickening of bronchial walls and may affect lung function. In the present study we set out to evaluate the relationship between small airway wall thickness and the lung function parameters in patients with asthma and COPD. The study was performed in 10 patients with asthma (4M/6F, the mean age 37+/-13 yr) and 12 patients with COPD (7M/5F, the mean age 57+/-9 yr) with stable, mild to moderate disease. The study group characteristics were based on clinical assessment and lung function testing (spirometry, body plethysmography, methacholine challenge test). All patients underwent chest high resolution computerized tomography with small bronchi (external diameter 1-5 mm) cross section measurements at five selected lung levels. The following parameters were measured in end-inspiratory scans: external (D) and internal (L) diameters, wall area (WA), percentage of the wall area (WA%), wall thickness (WT), and WT/D ratio (BWT). We found no significant correlations between airway wall thickness and spirometric parameters in either group. In the asthma group, the relationships between WA% and BWT, on the one side, and postbronchodilator residual volume, on the other, were noted (r=0.72; P<0.05 and r=0.72; P<0.05, respectively). In the COPD group, WA% related with airway resistance (r=0.72; P<0.05). The correlations between WA% and PC(20) (r=-0.61; P<0.05) and BWT and PC(20) (r=-0.72; P<0.05) were found in the COPD group. There was also a relationship between WA% and airway resistance (Raw) (r=0.72; P<0.05) and BWT and Raw (r=0.45; P=0.1). The number of pack-years correlated with WA and WT in COPD patients. In conclusion, the study shows that the thickening of airway wall in asthma is reflected by an increase in the indices of air trapping and in COPD this thickening results in a higher airway resistance and responsiveness. In COPD, the thickening of airway wall also is related to exposure to tobacco smoke.