This study investigated night-to-night variation in periodic leg movements in sleep (PLMS). PLMS are common in the elderly, but their mechanism and significance are not understood. Forty-five aged individuals (means = 69.7 years) were studied polysomnographically for 2 or 3 nights with surface electrodes placed above the anterior tibialis. Results indicated that PLMS varied considerably from night to night within individuals, though there was not a significant difference between nights for the entire group. Some evidence indicated that individuals with less severe sleep complaints showed greater nightly variation. The nightly variation in PLMS was over four times as large as the nightly variation in breathing disturbance in sleep, another condition common in the sleep of the aged. These data suggest that studies relating PLMS to other key variables (e.g. symptoms of disturbed sleep) should rely on multiple nights of data or, if single night data are used, be particularly careful to replicate findings across samples.