Twenty-three patients with hemifacial spasm were studied clinically and electrodiagnostically. Seven patients had mild facial weakness. All patients had clinical evidence of synkinesis, which often varied considerably. Facial nerve conduction and blink reflex latencies were normal. Facial synkinesis could be measured objectively on the involved side in all patients by simultaneously recording from the orbicularis oculi and orbicularis oris muscles at the time of supraorbital nerve stimulation. Using this procedure, synkinesis was also observed in association with aberrant regeneration after Bell palsy but was not seen in other movement disorders involving the face. The demonstration of synkinesis and its variability in hemifacial spasm can be of value in differentiating hemifacial spasm from other movement disorders affecting the face and provides further insight into its pathogenesis.