PURPOSE: We performed a retrospective study to investigate seizure, EEG, social and cognitive outcome in adult LGS subjects. METHODS: We retrospectively evaluated 27 LGS patients aged 40-59 years. We assessed in particular the evolution of different seizure types and EEG findings, as well as cognitive and social outcome. RESULTS: During the early stages of the disease, all patients presented tonic seizures (TS) during wakefulness and sleep, 20/27 had atypical absences (AA), more rarely other seizure types. EEG showed slow background activity in 21/27 patients, diffuse slow spike-wave discharges (DSSW) during wakefulness in 22/27, and bursts of diffuse fast rhythms (DFR) in sleep in all patients. At last observation, 11 patients only had TS during wakefulness, but all still presented TS during sleep; AA persisted in 6 patients. EEG showed normal BA in 12/27 patients; only 7/27 still presented DSSW. On the contrary, sleep EEG showed the persistence of DFR in all. A moderate to severe cognitive impairment was observed in 26/27 patients. CONCLUSIONS: In adult LGS patients TS during sleep remain the major seizure type; moreover, a standard waking EEG may be normal. Thus, polysomnography represents the most important mean of investigation also in adult LGS patients.