Fifteen cases of lissencephaly were studied and the literature reviewed. The authors conclude that the clinical findings of lissencephaly in infancy are non-specific, consisting of developmental delay and hypotonia. While the CT scan establishes the diagnosis, it may also be strongly suggested by an EEG showing 'major fast dysrhythmia', characterized by abnormally rapid, very high-voltage activity, predominantly in the alpha and beta frequency bands. Some possible mechanisms for this highly suggestive EEG pattern are proposed.