The incidence of non-convulsive epilepsy with electroencephalographic features of Lennox syndrome is related in a 4 years old patient treated with diphenyl-hydantoin. The child had convulsive epilepsy associated with focal multiple spikes in the electroencephalogram at the beginning of the treatment. Based on the observation of transitional forms of benign and severe childhood epilepsies the role of the drug in the establishment of severe yatrogenic epilepsy is discussed.