There were several reports in Japan on cases that had characteristic features of what is now called benign adult familial myoclonic epilepsy (BAFME). Patients with BAFME often had irregular fine dysrhythmic involuntary movement, which became evident on outstretched extended arms. One of the characteristics of BAFME was giant somatosensory evoked potential (G-SEP) with enhanced long-loop cortical reflexes and premovement cortical spikes by the jerk-locked averaging method, suggesting hyperexcitability of cerebral cortex. Differential diagnosis includes a variety of progressive myoclonus epilepsy, hereditary tremor, and idiopathic epilepsies such as juvenile myoclonic epilepsy (JME) and grand mal on awaking. Careful history-taking reveals that the BAFME family history had a dominant hereditary pattern of almost complete penetrance, which is unusual for idiopathic generalized epilepsy (IGE). The presence of involuntary hand tremor with seizure strongly suggests the diagnosis of BAFME.