The clinical efficacy of the trihexyphenidyl was investigated in 100 patients with movement disorders. The study group consisted of 54 women and 46 men. Their ages ranged from 18 to 70 years, and their duration of illness varied from a few months to 36 years. Each patient had a videotape of the movements and a neurological examination, before administration of the drug, at the time of maximum or effective dosage, and one week after withdrawal from trihexyphenidyl. The drug was administered at an initial total daily dose of 2 mg and gradually increased to a total daily dose of 60 mg over a period of 4-6 weeks. Improvements were rated both clinically and from the videotapes. Three groups of movement disorders demonstrated a significant response to trihexyphenidyl: (1) dystonia 37%; tonic torticollis demonstrated a significantly better response than the clonic variant (80% vs. 22%). (2) rhythmic-oscillatory movements of brainstem-cerebellar origin (palatal myoclonus, pendular nystagmus, facial myokymia) 90%; (3) cerebellar tremor 75%. Among 32 responders, 17 (56%) continued taking trihexyphenidyl beyond 24 months. Side effects consisted of dryness of the mouth, jitteriness, stomatitis, blurred vision, and forgetfulness.