Asterixis was studied in nine patients, using a new electrophysiological technique: silent period locked averaging (SPLA). There were two types of electromyographic (EMG) silence in the movements clinically judged as asterixis. The jerky movement in one of the two types might be caused by the silent period after the subclinical cortical myoclonus. SPLA would be useful for studying asterixis as well as other EMG silences.