Thyroarytenoid injection of botulinum toxin is the therapy of choice in spasmodic dysphonia. However, there is no convincing evidence as to whether unilateral or bilateral injections are to be preferred. For this reason, a prospective study was designed in which voice quality, duration of effect, and side effects were assessed. Twenty-seven patients with adductor spasmodic dysphonia were treated with percutaneous injections of botulinum toxin. The first treatment consisted of injection of 5 units in the left thyroarytenoid muscle. The second treatment, 2.5 units in both sides, took place when the effect of the first procedure had completely ceased. All patients underwent both procedures. By means of self-rating scales, effects and side effects were assessed over at least 3 months. There was no difference between the procedures in duration of voice improvement, nor in the occurrence of breathy dysphonia. After a bilateral injection, statistically more patients reported swallowing problems. However, most patients preferred the bilateral injection, in spite of more and longer-lasting side effects.