Both Obstructive Sleep Apnea Syndrome (OSAS) and epilepsy are prevalent conditions in general population. Recent publications have reported a higher prevalence of OSAS in clinical series of epileptic patients,suggesting that untreated OSAS may contribute to a worse control of epilepsy.Since OSAS is often underdiagnosed in general population,the aim of our study was to determine OSAS prevalence and severity in a large clinical setting of unselected epileptic patients referred for polysomnographic (PSG) evaluation. We evaluated 100 consecutive epileptic patients (mean age 43.4, 55 men),referred to Sleep Unit for PSG recording due to the following reasons: epileptic syndrome diagnosis (47%), nocturnal seizures (19%), OSAS suspicion (12%), drug resistant epilepsy (9%),isolated convulsive seizure (7%), and parasomnia (6%). All patients underwent attended video-PSG. Patients with an Apnea–Hypopnea Index (AHI ) ⩾ 5 were classified as having OSA. Prior to PSG evaluation demographic and clinical data were recorded (epileptic syndrome and number and type of antiepileptic drugs). The overall prevalence of OSAS in our series was 51%, with a mean AHI of 22.4 (SD 17.4). Among OSAS patients, 49,1% had mild OSAS, 23.5% moderate OSAS and 27% severe OSAS,with significant worse nocturnal saturation indices when compared with no-OSAS group. OSAS was confirmed in all patients with previous clinical suspicion, and was also diagnosed in 39 patients with no suspected OSA prior to the PSG evaluation (21 mild, 9 moderate and 9 patients with severe OSAS).Compared with no-OSAS group, OSAS patients were older (mean age 51.8 vs 34.7), with a higher BMI (28.1 vs 23.8) and mostly men (36% vs 19%, p < 0.05). Sleep architecture showed a higher percentage of non-REM sleep in OSAS patients ( p < 0.05), with no differences in the efficiency or REM sleep. No significant differences were observed in the Epworth Sleepiness Scale (ESS) or number of AEDs at the time of PSG. A high prevalence of OSAS was observed in our clinical setting of epilepsy patients, especially among older men with a higher BMI. A large number of patients (39%) were diagnosed of OSAS with no prior clinical suspicion of nocturnal respiratory disturbance. Clinical screening of OSAS in settings of epileptic patients may be needed in order to diagnose maybe a potential and modifiable risk factor for epilepsy. We thank Dr. Romero, Dr. Sampol, Dr. Jurado and Dr. Ferré for their support.