Botulinum toxin has been previously reported to be successful in the treatment of spasmodic dysphonia. To objectively document results, 11 patients with adductor spasmodic dysphonia who received unilateral treatment of the thyroarytenoid muscle were studied. Acoustic analyses and airflow rates during sustained phonation and flexible videoendoscopy were performed prior to, 1 week and 1 month after injection. 1. Acoustic parameters demonstrated significant voice improvement, although abnormal characteristics remained. 2. Mean airflow rates were increased 1 week after injection with almost normal values 1 month later. 3. Videolaryngoscopy showed an effective reduction of intrinsic laryngeal muscle hyperfunction with less effect on extrinsic muscle activity. Interrelations between videolaryngoscopic rating scores, acoustic results and aerodynamic results are discussed.