The treatment of adductor spasmodic dysphonia using botulinum toxin A was conducted in 13 patients as a double-blind, placebo-controlled study. Patients were diagnosed independently by an interdisciplinary team consisting of speech pathologists, an otolaryngologist, and a neurologist. The toxin or saline was injected into each thyroarytenoid muscle under electromyographic and laryngoscopic guidance. Botulinum toxin A markedly reduced perturbation, decreased fundamental frequency range, and improved the spectrographic characteristics of the voice. Fundamental frequency and phonation time remained unchanged. Patients injected with botulinum toxin A noticed significant improvement in their voices in comparison with the placebo-treated group. Excessive breathiness of the voice occurred in two patients, and mild bleeding in one patient in the botulinum toxin A-treated group. Injection with saline resulted in edema of the vocal cord in one patient. Botulinum toxin A proved to be an effective and safe treatment of adductor spasmodic dysphonia.