SUMMARYThe incidence of febrile convulsions was the same in parents and in sibs of 208 children with febrile convulsions–9% or three times the prevalence in the population. The findings and the rate in second- and third-degree relatives were compatible with transmission by a single dominant gene with incomplete penetrance. Half the affected relatives with epilepsy were distant. Rate of epilepsy in parents and sibs, uncles, aunts, and cousins was not higher than the prevalence in the population. 3We found no evidence that some families might differ from others in the mode of transmission nor that some cases were nongenetic. 4] There was clinical and EEG evidence of a different degree of expressivity in some families. Probands with parents, uncles, aunts, or cousins with febrile convulsions had twice as high a rate of recurrence as children with no relatives with febrile convulsions. Probands with a relative with convulsions developed 3-per-second spike-and-wave paroxysms in the EEG twice as often as probands with a negative family history. More than half the probands with solely epilepsy in the family had severe or multiple natal or neonatal complications. When a relative had febrile convulsions, one-fifth of the probands had natal or neonatal complications. 6] Recurrent spontaneous convulsions persisted in 3 children (1.4%), all severely brain damaged. One was a congenital idiot, I had an intercurrent encephalitis, and I was mentally retarded.