STUDY OBJECTIVE: To determine the prevalence, risk factors, and impact on daytime sleepiness and hypertension of periodic leg movements of sleep (PLMS) with associated arousals in patients with obstructive sleep apnea (OSA). METHODS: A single-center retrospective case series of 798 consecutive patients who underwent diagnostic overnight polysomnography for suspected OSWe performed discriminant function analysis using clinical and polysomnographic variables to examine the relationship between PLMS (periodic leg movement arousal index > or =5 per hour) and potential risk factors, including OSRESULTS: Mean +/- SD age was 50 +/- 12 years, body mass index 32 +/- 8 kg/m2, Epworth Sleepiness Scale (ESS) score 11 +/- 5, and apnea-hypopnea index 31 +/- 26 per hour. Sixty-eight percent were men, 30% had systemic hypertension, and 19% were smokers. Ninety-two percent had OSA (apnea-hypopnea index +/- 5); 47% had PLMS; 44% had both OSA and PLMS; and among patients with OSA, 48% had PLMS. Significant predictors of PLMS, in order of importance, were number of predisposing medical conditions, age, number of predisposing medications, obesity, and OSMedical conditions that significantly predicted PLMS were depression, fibromyalgia, and diabetes mellitus. The ESS score and hypertension status were no different between those with both OSA and PLMS and those with OSA alone. CONCLUSIONS: One in 2 patients investigated for OSA has PLMS. Risk factors for PLMS include preexisting medical conditions-particularly depression, fibromyalgia, and diabetes mellitus-increasing age, predisposing medications, obesity, and OSThe combination of OSA and PLMS results in no greater subjective daytime sleepiness or prevalence of hypertension than OSA alone.