BACKGROUND: Much of South East Asia is experiencing an epidemiological transition. In Thailand, chronic disease has emerged and the prevalence of diabetes has tripled. As part of a large cohort study of the Thai transition to chronic disease, we gathered data on self-reported diabetes. Epidemiological studies commonly ascertain such data by self-report but the validity of this method has not been assessed in Thailand. Therefore, we aimed to investigate the validity of self-reported type 2 diabetes (T2DM) in Thai adults participating in the Thai Cohort Study (TCS).METHODS: Data were collected by mailed questionnaire from adults involved in the TCS, a nationwide community-based longitudinal health study of distance learning adult students enrolled at Sukhothai Thammathirat Open University. Participants were surveyed in 2005, 2009 and 2013. We sampled all participants with self-reported T2DM status (878 cases) for telephone interview with our study physician along with a random selection of 650 participants who self-reported not having diabetes in all three TCS surveys. These physician telephone interviews allowed us to validate self-reported questionnaire responses.RESULTS: Questionnaire self-report of diabetes slightly over-estimated the incidence of T2DM in this cohort; the overall proportion of confirmed T2DM cases was 78%. Participants with a consistent pattern of diabetes reporting at the 2009 and 2013 questionnaire follow-ups had the highest validity of self-reported responses (96%; 95%CI 92.9-99.1).The lowest proportion of confirmed T2DM cases was recorded among participants who reported diabetes in 2009 and not in 2013 (32%)(95%CI 22.6-41.4), mostly young women with transient (gestational) diabetes.CONCLUSIONS: Our results, derived mainly from young, educated Thai adults nationwide, show that self-reported doctor diagnosed T2DM is a feasible and acceptable method for assessing diabetes in epidemiological studies.