The Epworth Sleepiness Scale (ESS) is a self-administered eight-item questionnaire that is widely used in English speaking countries for assessment of daytime sleepiness in adults. The aim of this study was to investigate the reliability and validity of the ESS in the Turkish language. The Turkish version of the ESS (ESStr) was applied to 194 healthy controls and 150 consecutive subjects attending the sleep centre with symptoms of sleep-disordered breathing. Test-retest reliability of the ESStr was tested in a separate group of 30 subjects. The ESStr scores of 60 subjects with mild to severe obstructive sleep apnoea (OSA) were compared with the ESStr scores of 60 healthy controls matched for age, gender, and body mass index (BMI). Concurrent validity with the Functional Outcomes of Sleep Questionnaire (FOSQtr) was also assessed in 12 subjects. The questionnaire had a high level of internal consistency as measured by Cronbach's alpha (> or =0.86). The test-retest intraclass correlation coefficient was r = 0.81 (95% confidence interval: 0.64-0.90) (p < 0.001) and Spearman's correlation coefficient was r = 0.80 (p = 0.01). The control group had lower ESStr scores than subjects with sleep-disordered breathing (3.6 +/- 3 vs 12.6 +/- 6, respectively; p < 0.001). Subjects with mild sleep-disordered breathing also had lower scores of the ESStr than those with moderate and severe sleep-disordered breathing (10 +/- 6.2 vs 14 +/- 5. and 10 +/- 6.2 vs 16 +/- 5.4, respectively; both p < 0.05), but there were no significant differences between moderate and severe subjects with sleep apnoea. There were significant correlations between the ESStr and total FOSQtr and its subscales (r = -0.22 to r = -0.92; all p = 0.05). Factor analysis of item scores showed that the ESStr had only one factor. The ESStr is a reliable and valid measure of daytime sleepiness. These features and the simplicity of the ESStr make it a valuable measure for clinical management and research.