Population based twin panels containing 14,352 twin pairs established in Norway and Virginia were used to study the occurrence of febrile seizures and epilepsy at the Departments of Human Genetics and Neurology, Medical College of Virginia, Virginia Commonwealth University, Richmond, VA; and the Institute of Medical Genetics, Oslo, Norway. There was a history of febrile seizures reported in 257 pairs and a history of epilepsy in 1 or both members of 286 pairs of twins. Virginia twins reported febrile seizures (1.98%) slightly more often than Norwegian twins (1.61%). Norwegian twin pairs reported epilepsy (2.19%) slightly more often than Virginia twin pairs (1.66%). There were no significant differences in the concordance rates between Virginian and Norwegian twins for either febrile seizures or epilepsy (for febrile seizures, 0.33, in monozygotic twins and 0.11 in dizygotic twins; for epilepsy, 0.19 monozygotic and 0.07 dizygotic). The excess of concordant monozygotic twin pairs observed for both epilepsy and febrile seizures was significant and showed a factor of almost 3 to 1. 
COMMENT. The inheritance of febrile seizures is multifactorial. Siblings have approximately an 8-12% risk of also having febrile seizures. If the index child and 1 parent are affected, the risks to siblings are 30%-40% (50% if both parents are affected).