A investigation was done on EEG evolution of 9 patients with PLEDs. In all patients PLEDs eventualy disappeared, in general in a short period, less than 4 daysin 5 cases. In 4 cases evolution was to slow background activity, and to posterior normalization in 2. In 3 cases evolution was to delta activity and suppression bursts, and all the 3 patients died. In the last 2 patients PLEDs were replaced by localized paroxysmal activity, which was anteriorly registered in 1. PLEDs was found in acute dysfunction of CNS, and in epileptic patients in periods of increased seizure activity. These data suggest that a critical point of alteration of the normal neuronal activity is needed for appearance and maintenance of PLEDs. No correlation could be done between the evolution of PLEDs, always to disappearance, and clinical outcome, that was variable.