Most polysomnograms are performed because sleep-disordered breathing (SDB) is suspected, but periodic leg movements during sleep (PLMS) are frequent incidental findings, and their significance is not well understood. In a clinical series of 1,124 adult patients with suspected or confirmed SDB, we tested for an association between the rate of periodic leg movements and one important outcome, the severity of daytime sleepiness. Objective sleepiness was assessed by the Multiple Sleep Latency Test in all subjects, problem sleepiness by self-rating in 873, and subjective sleep propensity by the Epworth Sleepiness Scale in 201. Increased leg movements were associated with decreased objective sleepiness (p = 0.03) but explained less than 1% of the variance. When nocturnal arousals were scored (n = 321 subjects), rates of leg movements associated with arousals predicted less objective sleepiness (p = 0.008); rates of leg movements without arousals predicted nothing. The rates of leg movements showed no association with subjective problem sleepiness or sleep propensity. We conclude that incidental periodic leg movements during sleep are not associated with excessive daytime sleepiness, and therefore appear unlikely to contribute to this problem. Increased sleepiness may reduce the likelihood of arousal with each leg movement.