To obtain a comprehensive clinical description of periodic movements in sleep, the first 11 videotaped movements of nine patients were studied. These leg movements were characterized by active dorsiflexion of the ankle (91%), dorsiflexion and fanning of the small toes (82%), and dorsiflexion of the great toe (72%). Partial flexion of the knee and hip occurred in 28%. Simultaneous dorsiflexion of the ankle and small toe was almost always the initial change, either preceding or occurring simultaneously with extension of the great toe. Knee and hip flexion almost always followed foot movements. Tonic and/or clonic movements, usually about the ankle, occurred in 75%; plantar flexion of the ankle occurred in 21% at the end of a leg movement. Periodic movements in sleep were thus characterized on detailed videographic analysis by movements similar to the Babinski response. Since both the normally occurring nocturnal Babinski response and periodic movements in sleep are also almost entirely NREM related, the author proposes that periodic movements in sleep are due to the NREM loss of supraspinal inhibitory influences on the pyramidal tract and that the characteristic foot and leg movements are Babinski-type responses secondary to this.