Time-of-night patterns for periodic leg movement in sleep (PLMS) activity were assessed in 72 patients diagnosed as having PLMS (46 with only PLMS, 10 with narcolepsy and PLMS and 16 with sleep apnea and PLMS). Two distinct patterns of PLMS activity were identified. For one pattern, PLMS activity was high initially upon falling asleep and then decreased across the remainder of the night with the majority of the activity occurring during the first half of the night. For the second pattern, PLMS activity was relatively evenly distributed across the night with slightly greater activity in the middle of the night. These patterns were highly reliable across two nights of recording for group 1 and less so for group 2 even though there was significant variability between nights in the number of PLMS. Patients in group 2 were younger (except for those with sleep apnea) and were sleepier during the day. Implications of these findings for pharmacologic management are discussed.