Geniospasm is a hereditary disorder characterized by episodic, usually stress-induced involuntary trembling of the chin. Nineteen families from Europe and the U.S.A. suffering from this disorder have been described since 1894. We present three cases from two newly detected families. There was no evidence of any other nervous system abnormality, although abnormal EEG, sleep disorders, and involvement of other facial muscles have been described in rare cases. Geniospasm is transmitted as an autosomal-dominant trait with high penetrance. It must be differentiated from facial myokymia, palatal tremor, and essential tremor affecting facial muscles. Neurophysiological and molecular mechanisms of this peculiar disorder are unknown.