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Seventy-eight individuals with EEG records containing benign epileptiform transients of sleep (BETS) were identified among 7,400 records reviewed in our laboratory in a 6-year period. The records contained no other abnormality in 51 patients (65%). Genuine epileptiform discharges were found in the records of 19 patients; 14 had a history of epilepsy. Thirty-five patients (45%) had a proven history of epilepsy with antiepileptic drug (AED) therapy. In the records of these patients, the mean number of BETS per unit of time was significantly higher (11.88 +/- 2) than in the record of the rest of the laboratory population with BETS (6.89 +/- 0.9) (p less than 0.02). Among five conventional surface montages, ipsilateral ear referential montage (IERM) showed a significantly higher number of BETS per unit of time than did any other surface montage used in the study. Thirty-nine records (50%) were performed after sleep deprivation (SD). When only IERM was considered, SD records showed a significantly higher number of BETS per unit of time (7.36 +/- 1.1) than did non-SD records (3.89 +/- 0.69) (p less than 0.01). Our findings support the general consensus that individual BETS may be normal variants, but a high occurrence of BETS in the record should raise suspicion of underlying epilepsy.