BACKGROUND: A self-reporting questionnaire is the most convenient and cheapest way to assess medication adherence. A new 8-item self-reported Morisky Medication Adherence Scale (MMAS) has been developed and has shown better psychometric properties than the original 4-item Morisky scale. OBJECTIVE: To examine the validity, including convergent known-groups and construct validity, and reliability, including internal consistency and test-retest reliability, of the MMAS in Thai patients with type 2 diabetes. METHODS: The data were derived from a cross-sectional study. In a convenience sample, 303 type 2 diabetic outpatients were interviewed at the General Police Hospital in Bangkok, Thailand, between January and June 2007. Face-to-face interviews included MMAS, medication adherence visual analog scale (MA-VAS), and sociodemographic data. Medical records were reviewed for clinical data such as hemoglobin A(1C) (A1C) levels. RESULTS: Internal consistency reliability was moderate (Cronbach's alpha = 0.61), whereas the test-retest reliability of the MMAS was excellent (intraclass correlation coefficient = 0.83; p < 0.001). Concerning convergent validity, the MMAS had a high correlation with the 3-item Morisky scale (r = 0.77; p < 0.01) and a medium correlation with the MA-VAS (r = 0.57; p < 0.01). Regarding known-groups validity, a significant association between MMAS and A1C levels was found (chi(2) = 6.7; p < 0.05). The sensitivity, specificity, positive predictive value, and negative predictive value of the MMAS were 51%, 64%, 71%, and 43%, respectively. Our factor analysis showed that the MMAS had 3 dimensions including forgetting to take medications, stopping medications when feeling better or worse, and the complexity of the drug regimen. CONCLUSIONS: The 8-item MMAS can be a tool to aid in assessing medication adherence in diabetes. The poor sensitivity can be improved by increasing the number of the response choices and the cut-off score of the scale and by using specific words in some items. A modified Thai version of the 8-item MMAS may be needed.