We describe 11 patients affected by Landau-Kleffner syndrome (LKS) with a mean follow-up of 9 years and 8 months. EEG recordings during wakefulness, NREM and REM sleep showed a bitemporal electrical status epilepticus during sleep (BTESES) in all cases; four of them presented a shift from a BTESES towards an 'intercalated electrical status epilepticus during sleep' (IESES) accompanied by a global regression of cognitive and behavioural functions in 3/4 of cases. At the last observation, only 18.2% of cases presented a complete language recovery and mental retardation was evident in 63.6%. The prognosis of LKS in our cases may depend on the interaction of different negative factors such as onset of aphasia before 4 years, its duration for longer than 1 year, long-lasting duration and continuity without fluctuations of BTESES/IESES, probably preexisting mild speech delay. It is important for the prognosis to utilize antiepileptic treatment and possibly neurosurgical techniques to eliminate EEG paroxysmal abnormalities. At present, no similar cases with clinical-EEG evolution from LKS to electrical status epilepticus during sleep (ESES) have ever been described. Our observation demonstrates that LKS and ESES classified as different clinical-EEG syndromes represent two aspects of the same brain dysfunction and they may exist separately or pass one into the other with a change in the clinical-EEG picture. The common origin of the two syndromes is confirmed by recent functional brain imaging, neurophysiological and neurosurgical techniques.