A sleep EEG of 190 patients without sleep deprivation was recorded, followed by a sleep EEG after 24 h of sleep deprivation on the next day. The patients suffered from various types of epilepsy, in their routine EEGs no epileptic discharges were seen. Both sleep EEGs were recorded under the same antiepileptic drugs. A waking EEG was recorded immediately before each sleep EEG. The activation rates of epileptic activity in 52.6% (without sleep deprivation) and 53.2% (with sleep deprivation) of the patients showed no significant differences. Also on classifying the epileptic discharges no real difference was found between the 2 methods (generalized: 29.5 vs. 29.5%, generalized with lateral emphasis: 11.1 vs. 9.5%, focal: 12.1 vs. 14.2%). Only in the waking EEG, recorded immediately before the sleep EEG after sleep deprivation, a few more patients showed epileptic discharges (33.6 vs. 27.4%). Without there being any significant differences between the 2 methods there were some different results in comparing the EEG with the clinical findings: significantly more epileptic activity was shown in patients who had their first seizure before the age of 20 (55.6 and 55.6% vs. 26.3 and 31.6%), amongst females (59.8 and 61.9% vs. 45.2 and 44.1%), in awakening grand mal (= primary generalized tonic-clonic seizures, 76.5 and 70%) and in absences (69 and 72.4%). The higher activation rates in young subjects, in patients with a family history of seizures, with pathological neurological findings, mental retardation and delayed psychomotoric development in early childhood, were not statistically significant.(ABSTRACT TRUNCATED AT 250 WORDS)