Hemifacial spasm is characterized by unilateral, periodic, tonic contractions of facial muscles, thought to be caused by mechanical compression at the root-exit zone of the facial nerve. Electrophysiologic abnormalities such as ectopic excitation and synkinesis are typical. Although posterior fossa microsurgical nerve decompression is successful in bringing about relief of the spasm in most cases, it carries a risk to hearing. As an alternative treatment, 15 patients with hemifacial spasm were given a total of 41 sets of injections with botulinum A toxin, with a mean follow-up of 14.3 +/- 1.1 months. Relief of symptoms lasted a mean of 108.3 +/- 4.2 days. Mild transient lagophthalmos and ptosis were the only complications. Although the exact mechanism of its action and beneficial effect is speculative at this time, botulinum A toxin appears to offer an effective, safe alternative to more radical intracranial surgery for patients with hemifacial spasm.