One hundred patients were analyzed sequentialy regarding to the presence of periodic movements in sleep (PMS) or nocturnal myoclonus. The criteria for considering PMS was the presence of 5 or more myoclonic movements per hour of sleep. The 18 patients included in the PMS cathegory were compared with the remaining 82. Those with PMS had tendency to be older than the others. The movements were observed in stage 2 and less commonly in stages 3,1,4 and REM. Patients with PMS had generally higher number of arousals and Κ complexes. There was no difference between both groups regarding sex, history complaints, sleep efficiency, total sleep time, time in each stage and final diagnosis.