The Epworth sleepiness scale (ESS) is a widely used instrument in the subjective assessment of sleepiness. Although there are several translations into Spanish, their equivalence to the original version is questionable.To assess the reliability and validity of a true translation into Spanish of the ESS in Mexican population.The ESS was translated into Spanish with the use of standard translation methodology: forward translation, back translation and bilingual committee consensus. It was administered to six groups of subjects of the following categories: narcolepsy, obstructive sleep apnea syndrome (OSAS), major depression with OSAS risk, major depression without OSAS risk, good sleepers and insomniac patients without OSAS risk. Internal consistency and factorial structure of the ESS was estimated. In addition, a comparison between groups of the ESS scores was conducted.The ESS showed to be composed by only one factor and it also showed a high reliability coefficient (0.89). Likewise patients with narcolepsy or OSAS had the highest scores while good sleepers obtained the lowest scores.The true translation into Spanish of the ESS showed similar psychometric properties to the original version and, superior to previous Spanish adaptations. Therefore, the ESS is a reliable instrument for the assessment of sleepiness in our population.