To assess the efficacy of bilateral or unilateral botulinum treatments for spasmodic dysphonia we injected botulinum toxin (type A) into the thyroarytenoid muscle of 24 patients with adductor type spasmodic dysphonia. Eleven patients underwent unilateral procedures and 13 bilateral procedures. Samples of sustained phonation were analyzed acoustically by a computer-assisted method and the air flow rates determined. All tests were conducted 1 week before injection and 1 week and 1 month after treatment. With unilateral injection, improvements in acoustic parameters occurred as early as 1 week after treatment. With bilateral injections, only the voice break factor was significantly reduced after 1 week, while standard deviations of fundamental frequency, jitter, shimmer and signal-to-noise-ratios were reduced 1 month after treatment. In comparison with unilateral injections, the mean air flow rate was twice as high 1 week after bilateral injections, with no significant differences found 1 month after treatment. Clinically, both injection modes resulted in the reduction of laryngeal spasms as early as within 48 h after injection.