© 2015 Pothirat et al. Background: Chronic obstructive pulmonary disease (COPD) patients living in many countries are familiar with local dialects rather than the official language. We, therefore, compare the reliability and validity of the COPD assessment test (CAT) in Thai and northern Thai dialect versions, in stable COPD patients living in the northern part of Thailand. Methods: Atotal of 160 COPD patients were randomly selected for the evaluation of each dialect version of CAT(n=80). The internal consistency of all eight items and test–retest reliability were investigated by using Cronbach’s alpha coefficient and intraclass correlation coefficient (ICCC), respectively. The validity was evaluated by the degree of correlation with St George’s Respiratory Questionnaire (SGRQ) using Pearson’s correlation. The correlations of CATwith clinical parameters such as forced expiratory volume in the first second (FEV1), modified Medical Research Council scale (mMRC) dyspnea score, and 6-minute walk distance (6-MWD) were also evaluated. Results: The two versions of CATshowed high internal consistency reliability (Cronbach’s alpha coefficient of 0.82 and 0.76) as well as a high test–retest reliability (ICCC of 0.82 and 0.84) for Thai and northern Thai dialect versions, respectively. The test results revealed that the northern Thai dialect version had good correlation with SGRQ whereas the Thai version correlated only moderately. Conclusion: The two Thai versions of CATwere proven to be good clinical tools with high reliability and acceptable validity for assessing the quality of life of Thai COPD patients. However, the northern Thai dialect version is more suitable for evaluating COPD patients living in the northern part of Thailand.