PURPOSE: To investigate the reliability of the Turkish version of the self-assessment mornings-evenings questionnaire (MEQ). METHODS: A Turkish version of the MEQ, which was originally designed by Horne and Ostberg (1976), was given to 618 (291 women and 327 men) volunteers, aged 18 to 57 (23.0+/-5.4 years). Participants were recruited from the university student population and staff who were not night-shift workers. Exclusion criteria included sleep disorders and a history of medical and psychiatric diagnoses. The participants completed the questionnaire twice, 15-20 days apart. The reliability was assessed by Cronbach's and Pearson's r coefficient. RESULTS: MEQ scores were 51.9+/-9.0 (range 22-73) and 52.4+/-9.5 (range 21-72) for the 1st and 2nd applications, respectively, and did not differ significantly between the two applications. Of the 618 subjects interviewed, 149 were self-reported as morning types at the 1st application and 169 as morning types at the 2nd application; 79 as evening types at the 1st application and 87 as evening types at the 2nd application; and 390 as intermediate types at the 1st application and 362 as intermediate types at the 2nd application. Cronbach's a was 0.785 and 0.812 for the 1st and 2nd applications, respectively, and the test-retest reliability coefficient was 0.84 (p< 0.0001). DISCUSSION: The Turkish version of the MEQ shows high reliability. Nevertheless, further evaluation using different subject groups such as the elderly, athletes and shift workers is required to confirm the reliability and validity of the Turkish version.