The effect of clonazepam, taken per-os and in very progressive doses, has been studied by the authors on fourteen children afflicted with evolutive epileptic encephalopathies and still presenting, in spite of heavy anti-epileptic polychemio-therapies, alarming clinical and electrical manifestations. Clonazepam proved to be an efficient contributive factor in the course of a curing process since, in 70% of the cases, it produced quite a visible amelioration. Its efficient action seems to last longer than that of other diazepines since half of their patients have now been treated with it for more than a year with, as a result, a constant clinical and electrical amelioration. As a last point, it appears to be a very useful drug when dealing with type of rebellious epilepsies since, when combined with other benzodiazepines, seldom side-effects and no "crossed resistances" can be observed.