A human case of rhythmic myoclonus of the quadriceps related to involvement of the crural nerve by a Schwann cell sarcoma of the thigh is reported. The rhythm of involuntary contractions varies between 35 and 300/min. The myoclonic jerks occur rhythmically in the quadriceps only during full extension of the knee. The jerks are inhibited during lengthening of the quadriceps by active or passive flexion of the knee. Clonazepam is effective in controlling the myoclonus. Myoclonus is the consequence of a response of .alpha.-motoneurons of the spinal cord to the excitation of Ia fibers of the crural nerve by the tumor. This activation appears to be enhanced by the descending excitation pathway and inhibited by activation of Golgi tendon organs of the quadriceps.