In symptomatic epilepsies due to chromosomal aberrations, epileptogenesis may be either the direct consequence of deletion or duplication of a gene causing seizures or may have a more complex etiology caused by the disturbance of the interaction of several genes and environmental factors. We report on a brother and a sister with trisomy 19q13.3-->qter who present different epileptologic features and discuss epileptogenesis in this syndrome with respect to genes known to be located on the distal part of chromosome 19q. Both patients share mental retardation and several dysmorphic features. The boy was hypoxic at birth and showed an extremely delayed psychomotor development. The girl, however, had no significant neonatal problems, and her psychomotor development was better. Although the male had an abnormal EEG in childhood, his first partial seizures occurred only as late as at age 31 years. He subsequently became seizure-free with carbamazepine (CBZ). In contrast, the girl already suffered from absence-like seizures during childhood and became seizure-free under ethosuccimide (ESM). A photoparoxysmal response, however, is still visible in her EEG. The difference between the epileptologic features in these siblings points to epileptogenic mechanisms placed far downstream on the way from genotype to phenotype. The photoparoxysmal response--otherwise a facultative finding in genetically determined epilepsies--in the EEG of the sister, however, points to a closer relationship between the duplicated genes and epileptogenesis. The fact that genes encoding potassium channels are located on 19q13.3-q13.4 may also support the latter assumption.