Background: A chronic obstructive pulmonary disease (COPD) assessment test (CAT) has recently been developed as a short and simple method for assessing the quality of life in COPD patients. The object of this study was to assess the usefulness of the Korean version of the CAT for assessing COPD patients in an outpatient clinic. Methods: The study included 60 COPD patients in a stable state from an outpatient clinic. The authors investigated the frequency of acute exacerbation during aprevious year through reviewing medical records. We evaluated the spirometry test, a 6-min walk distance test, and obtained the MMRC dyspnea scale, the Korean version of the CAT, and the BODE index at the time of visit. To assess the usefulness of the CAT, correlations between the CAT and other methods were evaluated. Results: The mean age of patients was $68.3{pm}8.6$ years and 95% of patients were male. There was a significant correlation between the CAT score and $FEV_1%$ (r=-0.323, p=0.012), the frequency of acute exacerbation (r=0.292, p=0.024), the MMRC dyspnea scale (r=0.554, p<0.001), the BODE index (r=0.380, p=0.003), and 6 MWD (r=-0.372, p=0.004). The mean CAT score increased according to the GOLD stage (stage 1, $10.7{pm}4.5$; stage 2, $13.1{pm}7.9$; stage 3, $16.3{pm}6.2$; stage 4, $16.5{pm}14.8$; p=0.746). Conclusion: The CAT was shown to be useful for the assessment of COPD severity. Therefore, the CAT is an easily applied and simple method for assessing COPD severity in an outpatient clinic.