Myoclonus, colloquially known as "the jerks," has remained an enigmatic clinical entity for more than 70 years, despite the frequent use of this term by neurologists. During this time, little emphasis has been placed on this symptom with regard to its clinical implications and prognostic signifiicance. It is the purpose of this paper to review the records of patients exhibiting myoclonus, exclusive of patients with the rhythmic type involving the velum palatinum and the pharynx, and to report on follow-up studies on these patients. The pertinent literature will be reviewed, and the clinical and prognostic significance of myoclonus will be elaborated. Definition Myoclonus may be defined as an involuntary, repetitive, desultory, instantaneous, irregular contraction of a group of muscles or, occasionally, a single muscle. Such contractions must be differentiated from the fasciculations seen clinically and in electromyographic studies. It may be difficult to recognize this clinical symptom, and the diagnosis