Motor involvement in acute herpes zoster is considered rare, but its incidence is unknown. In a sample of 40 patients with acute herpes zoster of varying severity, an abnormal electromyogram (EMG) (fibrillation, positive waves, high-frequency discharges) was found in 21 (53%), suggesting extension of inflammation to the anterior horn and/or anterior motor roots. In the majority of patients these changes were not confined to the segment invaded by the rash but were widespread, extending several segments cranially and caudally, and both ipsi- and contralaterally. In 5 (13%) patients these changes became more extensive on repeat EMG over a period of months. There was no association between severity of rash, pain, postherpetic neuralgia, and EMG changes. We conclude that widespread subclinical motor involvement is relatively common in herpes zoster, may last for months, and is easily detectable by EMG.