OBJECTIVE To evaluate the effects of eszopiclone on measures of respiration and sleep using polysomnography in patients with mild to moderate obstructive sleep apnea syndrome (OSAS). METHODS This double-blind, randomized crossover study included patients (35-64 years) with mild-to-moderate OSAS [apnea and hypopnea index (AHI) range 10 and 40]. Patients received either eszopiclone 3mg or placebo for two consecutive nights, with a 5-7 day washout between treatments. Continuous positive airway pressure (CPAP) was not allowed on nights in the sleep laboratory. RESULTS The primary endpoint, mean total AHI, was not significantly different from placebo (16.5 with placebo and 16.7 with eszopiclone; 90% confidence interval (CI) -1.7, 1.9). No significant differences in total arousals, respiratory arousals, duration of apnea and hypopnea episodes, or oxygen saturation were noted. Significant differences in spontaneous arousals (13.6 versus 11.4 for placebo and eszopiclone, respectively; 90% CI -3.7, -0.7), sleep efficiency (85.1% and 88.4%; p=0.0075), wake time after sleep onset (61.8 and 48.1 min; p=0.0125), and wake time during sleep (55.9 and 43.2 min; p=0.013) were noted after eszopiclone treatment. Eszopiclone was well tolerated. CONCLUSIONS In this pilot study, eszopiclone did not worsen AHI, and it improved sleep maintenance and efficiency. Further study is warranted to determine whether eszopiclone could improve CPAP compliance or next-day function in patients with OSAS.