We observed abnormal involuntary movements, involving principally the facial and neck muscles, in 23 patients with stuttering. These movements were similar to involuntary movements seen in distinct dystinic syndromes. There was a history of stuttering in the first degree relatives of six patients. The association of stuttering with degenerative neurologic disorders and focal brain lesions, cerebral blood flow changes in patients with developmental stuttering, its occurrence as a side effect of centrally acting drugs, induction and alleviation of stuttering by mechanical perturbation, or by electrical stimulation of the thalamus, a strong genetic predisposition with male preponderance, and the statistically significant occurrence of stuttering in the family history of patients with idiopathic torsion dystonia suggest an organic basis for developmental stuttering. These findings and the reported similarities between the involuntary movements associated with stuttering and dystonic involuntary movements support the hypothesis that stuttering is a form of segmental or focal action dystonia.