Spasmodic dysphonia (SD) is a focal dystonia of laryngeal muscles seriously impairing quality of voice. Adductor SD (ADSD) is the most common presentation of this disorder that can be identified by specialized phoniatricians and neurologists firstly on a clinical evaluation and then confirmed by videolaryngoscopy (VL). Botulinum toxin (BTX) injection with electromyographic guidance in muscles around vocal cords is the most effective treatment. Voice Handicap Index (VHI) questionnaire is the main tool to assess dysphonia and response to treatment. Objective of this study is to perform VL and voice spectrography (VS) to confirm the efficacy of BTX injections over time. 13 patients with ADSD were studied with VHI, VL and VS before and after 4 consecutive treatment with onobotulinumtoxin-A. For each treatment vocal improvement was proved by a significant reduction of VHI score and increase of maximum time phonation and harmonic-to-noise ratio while VL showed the absence of spasm in most of patients. No change of the response to BTX was found between injections. This study supports the efficacy of the treatment of SD with BTX with objective measurements and suggests that the efficacy of recurring treatments is stable over time.