In this study, we report the cases of five unrelated patients with Klinefelter's syndrome and seizures or EEG epileptiform abnormalities; the karyotype was 47,XXY in four, and 47,XXY/46,XX in one. They were aged 13-25 years and followed up both clinically and by means of EEG. Two of the patients had epilepsy, one had only one isolated generalized tonic-clonic seizure, one had febrile convulsions and one presented focal epileptiform EEG abnormalities without seizures. In two of the patients, it was possible to classify the epilepsy (childhood epilepsy with occipital paroxysms and cryptogenic or symptomatic generalized epilepsy). Although the electroclinical patterns appeared to be rather heterogeneous in our patients, it is possible to infer the relative good evolution of seizures in Klinefelter's syndrome.