BACKGROUND Botulinum toxin A (BTX) is the currently preferred symptomatic treatment for primary hemifacial spasm (HFS), but its long-term efficacy and safety are not known. OBJECTIVE To assess the long-term effectiveness and safety of BTX in the treatment of primary HFS. DESIGN Retrospective review of medical records of the 1st and 10th years of treatment. SETTING Outpatient clinics of 4 Italian university centers in the Italian Movement Disorders Study Group. PARTICIPANTS A series of 65 patients with primary HFS who had received BTX injections regularly for at least 10 years. MAIN OUTCOME MEASURES Mean duration of improvement and quality of the effect induced by the preceding treatment (measured using a patient self-evaluation scale) and occurrence and duration of adverse effects in the 1st and 10th years of treatment. RESULTS Using a mean BTX dose per treatment session similar to that used by others, we obtained a 95% response rate and an overall mean duration of improvement of 12.6 weeks during year 1. The effectiveness of BTX in relieving the symptoms of primary HFS, as measured by the response rate and average duration of improvement, remained unchanged in the 1st and 10th years. Patients needed statistically similar BTX doses in the 1st and 10th years. The rate of local adverse effects (including upper lid ptosis, facial weakness, and diplopia) diminished significantly in the 10th year of treatment. CONCLUSION Treatment with BTX effectively induces sustained relief from symptoms of HFS in the long term, with only minimal and transient adverse reactions.