The objective of study is to determine the thickness of bronchial walls of clinically diagnosed asthmatic cats using high resolution computed tomography (HRCT) compared to that of healthy cats. The bronchial walls and pulmonary arteries were measured in healthy 16 cats and clinically asthmatic 4 cats. The bronchial walls and pulmonary arteries were measured under general anesthesia with positive pressure inspiration using HRCT. In healthy and asthmatic cats, bronchial lumen to the artery ratio (BA ratio), the ratio of bronchial wall thickness to bronchial diameter (TD ratio) and ratio of bronchial wall thickness to pulmonary artery (TA ratio) were measured. The mean BA ratio, TD ratio and TA ratio in healthy cats were 0.86 ± 0.12, 0.18 ± 0.02 and 0.25 ± 0.05, respectively. Under the same condition, the mean BA ratio, TD ratio and TA ratio in asthmatic cats were 0.93 ± 0.21, 0.22 ± 0.24 and 0.37 ± 0.06. The TD ratio and TA ratio in asthmatic cats were significantly higher than healthy cats (P<0.001). BA ratio was not significantly different in both groups (P>0.05). The evaluation of bronchial wall thickness by HRCT could be useful for diagnosis of disease of bronchial wall thickening, such as feline asthma.