Facial muscle responses in patients with hemifacial spasm undergoing microvascular decompression operations were recorded. Two peripheral branches of the facial nerve were stimulated and the electrical responses of muscles innervated by these branches were studied to see how the lateral spread of activity that is known to be present in these patients was affected by decompressing the facial nerve. In some of the patients the hemifacial spasm ceased when the dura mater was opened, in some it ceased when the arachnoid was opened, and in others the spasm persisted until the offending vessel was dissected away from the nerve. The lateral spread of activity elicited by antidromic stimulation of a branch of the facial nerve was less affected by opening of the dura mater or arachnoid: it usually persisted until the blood vessel that had been compressing the facial nerve was removed and reappeared when the vessel that had been compressing the facial nerve was allowed to slip back onto the nerve. This seems to indicate that microvascular decompression of the facial nerve is effective in alleviating hemifacial spasm because it removes the actual cause of the disorder rather than simply causing local injury to the nerve as a result of the procedure.