Ramelteon is a selective MT(1)/MT(2)-receptor agonist indicated for insomnia treatment. Because it has no depressant effects on the nervous system, it is not expected to affect the control of breathing. The potential effects of ramelteon on apneic and hypopneic events and arterial oxygen saturation (SaO(2)) in individuals with obstructive sleep apnea were assessed. In this double-blind, randomized, crossover study, 26 adults with mild to moderate obstructive sleep apnea received ramelteon 16 mg and placebo for one night each, with a 5- to 12-day washout period between treatments. Treatments were administered 30 min before habitual bedtime. Respiratory effort was monitored using respiratory inductance plethysmography, SaO(2) was measured by pulse oximetry, and sleep onset and duration were measured by polysomnography and post-sleep questionnaire. Post-sleep questionnaire also measured next-day residual effects. The primary measure was apnea-hypopnea index. Apnea-hypopnea index was similar in ramelteon and placebo groups (11.4 vs 11.1, respectively; CI = -2.1, 2.6, P = 0.812). Ramelteon had no effect on the number of central, obstructive, or mixed apnea episodes. No significant differences were observed in SaO(2) for the entire night between ramelteon and placebo (95.1 vs 94.7%; P = 0.070). Ramelteon did not meaningfully affect sleep when evaluated by polysomnography and post-sleep questionnaire. Compared with placebo, ramelteon had no significant effect on next-day residual effects. Adverse events were reported by three subjects in the ramelteon group: headache (n = 2) and urinary tract infection (n = 1). No adverse events were reported with placebo. Ramelteon was well-tolerated and, as expected, did not worsen sleep apnea when administered to subjects with mild to moderate obstructive sleep apnea.