The electroencephalographic features and their evolutional changes with age were described in three cases of lissencephaly syndrome diagnosed by CT scan. The case with more severe lissencephaly displayed very similar EEG findings. In early or middle infancy when infantile spasms began, EEG showed very high amplitude (more than 400 microV) slow waves mixed with sharp theta-waves. In their clinical course, they showed extreme spindles and in late infancy, the EEG revealed a tendency towards bilaterally synchronous discharges of high amplitude sharp and slow waves. On the other hand, milder forms of lissencephaly showed hypsarrhythmia in early infancy. In the late infancy the EEG showed bisynchronous sharp and slow waves of more than 200 microV. The anomaly ranging from agyria to pachygyria seems to be closely associated with varying EEG abnormalities from extremely high voltage hypsarrhythmia to focal spikes. The very high voltage of hypsarrhythmic patterns and the very low frequency of sharp wave discharges seem to be typical in the most severe lissencephaly or agyria