Of 66 patients with Lennox-Gastaut syndrome, 15 exhibited the first clinical manirestations after 6 years of age. Twelve of these were followed for a mean period of 2 years and 6 months, with neurological evaluations at about 2-month intervals. A review of the literature showed that, in 20% of the cases studied, the Lennox-Gastaut syndrome begins after the 6 th year of life. Clinical evaluation allowed us to find out that our patients exhibited serious neurological damage preceding by several years the onset of the syndrome. The psychometric evaluation showed extensive involvement of the intellectual level, in contrast with the data in the literature on late-onset Lennox-Gastaut syndrome.