Epileptic activity was recorded in the waking and sleep EEG of 62.5% of 80 siblings of 38 patients with absence seizures. Epileptic discharges were noted in waking only in 8.7%, in waking as well as sleep in 28.8%, and in sleep only in 25%. Generalized, partly irregular, and slow spike-wave complexes were found, twice with lateral emphasis. Spike-wave complexes were recorded in 72% of 50 siblings of patients with idiopathic absence and in 46.7% of 30 siblings of patients with symptomatic absence. One epileptic discharge was observed every 108.6 s on the average, without striking differences between siblings of patients with idiopathic (99.7 s) and symptomatic absence (119.3 s). Without any differences between siblings of children with idiopathic and symptomatic absence, the most epileptic discharges were activated in sleep stages C and D, followed by stages A and B. The highest activation rate was observed in the 7-14-year-old group (73.5%) and to a somewhat lesser degree in the group between 15 and 20 years of age (66.7%); fewer epileptic discharges were recorded in younger (25%) and older patients (28.6%). The higher activation rates in the male sex were significant only in siblings of patients with idiopathic absence. Although only five patients (13.2%) were photosensitive, a photosensitivity was found in 24% of siblings of children with idiopathic absence and in 20% of siblings of patients with symptomatic absence. Three siblings of patients with idiopathic absence also had absence seizures; in one of them a febrile seizure occurred at an earlier age. All of them showed generalized spike wave discharges in waking as well as sleep. Occipital theta delta activity with generalization was observed more frequently in siblings of patients with idiopathic absence (82.2%) than in those of patients with symptomatic absence (63.6%). Our waking and sleep EEG recordings prove that concerning etiology-genetic factors play a striking role in idiopathic absence, but are also of considerable significance in the symptomatic types.