A longitudinal linguistic analysis of aphasic disorder in a 15-year-old boy affected by Landau-Kleffner syndrome followed since the age of 6 is reported. The phonological, morphosyntactic and lexical levels of verbal deficits have been evaluated by means of collected samples of spontaneous language and a battery of linguistic tests. The clinical course has fluctuated with improvement and worsening of aphasia and epilepsy; at the end of the follow-up the boy was seizure-free and a medium-degree disturbance in language production and comprehension was present. The results of the linguistic evaluation suggest that the aphasic disturbance was related to a deficiency in phonological decoding which leads to phonological, morphosyntactic and lexical disturbances. A temporal relationship between the electroclinical picture and the aphasia has been observed: the persistent improvement in linguistic performances took place only after the disappearance of the seizures and of the EEG epileptic anomalies during sleep.