The authors used the definition of the Lennox-Gastaut syndrome (LGS) adopted by the Commission on Classification and Terminology of the International League against Epilepsy. Three hundred and thirty eight patients with childhood LGS have been followed until adulthood; 62.4 p. 100 had an unfavourable outcome. All began their LGS in childhood or infancy (between the ages of 1 and 8.80 p. 100 before the age of 4). In 46.9 p. 100 of cases, complete LGS persisted in the adult. Most cases were apparently primitive. In 15.6 p. 100 of cases, generally symptomatic, the LGS disappeared but an often severe, mostly multifocal epilepsy persisted. 37.6 p. 100 of cases had a more or less favourable outcome. In these cases, the LGS had often begun later (between 7 and 11 years of age) and lasted for less (between 2 and 6 years). Some patients (20.4 p. 100) still had fairly rare partial seizures and neurological or psychiatric symptoms. These cases were mostly symptomatic, 17.4 p. 100 of cases seemed to have been nearly completely cured. These were cases where the LGS had followed another type of epilepsy, mostly of the idiopathic generalized type. Fourty four patients with no prior epilepsy presented with LGS appearing between the ages of 13 and 23. They were divided into 2 groups: in 31 patients, the LGS was associated with focal signs. In all these cases, the evolution was for the worse, whatever the age at onset. In 13 cases, there were no focal signs. In these, there was a different prognosis between those with an onset between the ages of 13 and 15, where the entire syndrome persisted and the evolution is for the worse, and those with an onset after the age of 15, where the outcome was better.