Three patients with idiopathic hemifacial spasm were studied clinically and electrophysiologically before and after injections of botulinum toxin into the involved periocular and facial muscles. The spasms were improved for approximately 3 months, and the effect was repeatable on reinjection. The spasms diminished only as long as the muscles were clinically weak, and spasms were observed electromyographically even though therapy eliminated the clinical spasms. Uninjected muscles continued to have spasms. Transmission of excitation from the zygomatic branch to the marginal mandibular branch of the facial nerve and vice versa in all patients was unaltered after therapy, but the amplitude of the response was decreased. The efficacy of botulinum toxin in hemifacial spasm appears to be related to the production of muscle weakness; there is no demonstrable effect on phenomena believed to be ectopic excitation or ephaptic transmission in the facial nerve.