AbstractThe electro‐clinical characteristics of childhood epilepsy are known to appear during the development of the brain and to change their features with age. Accordingly, developmental viewpoints are indispensable to researches on childhood epilepsy.Age‐dependent epileptic encephalopathy has the most remarkable developmental characteristics among childhood epilepsies.Early‐infantile epileptic encephalopathy with suppression‐burst (EIEE), the West syndrome and the Lennox‐Gastaut syndrome, which constitute age‐dependent epileptic encephalopathy, have many common features. Each of these three syndromes, however, has its own distinctive clinical and electro‐encephalographic specificities, and shows mutual transition with age.Results of developmental studies on the West syndrome are described, confirming it as one type of age‐dependent epileptic encephalopathy.1) On the basis of the long‐term follow‐up study of 14 cases of EIEE, sequential transition from EIEE to the West, and further to the Lennox‐Gastaut syndrome was observed, with reference to critical changing period, transitional patterns and prognoses.2) Analytical follow‐up study of 83 cases of the West syndrome, divided into idiopathic and symptomatic groups showed its evolution into the Lennox‐Gastaut syndrome to be important for its prognosis, with reference to the various factors re