The importance of predisposing factors in the pathogenesis of photosensitive epilepsies is examined. The morbidity of seizures is greatest in the female family members, and particularly on the maternal side. The morbidity is more frequent in the families of probands with seizures than in families of probands without seizures (4.4% and 1.6% respectively). The electroencephalographic examinations revealed a photoconvulsive response in 26.2% of the siblings and 6.8% of the control children (p <0.0005). The phenomenon occurred with maximum penetrance between the ages of 6 and 15 years. Besides photosensitivity, other genetic factors seem also to play a role in the triggering of seizures. The photoconvulsive response is regarded as a symptom of dispositional susceptibility to convulsions of the centrencephalic type. A symptomatic photosensitivity caused by organic brain lesions would appear to be a rare occurrence. Anhand von familienanamnestischen Erhebungen und elektroenzephalographischen Geschwister-Untersuchungen wird die Bedeutung dispositioneller Faktoren in der Pathogenese photosensibler Epilepsien untersucht. Die Untersuchung geht von 100 photosensiblen Probanden aus, von denen 68% an zerebralen Anfällen verschiedenen Typs leiden.