Genetic generalized epilepsy with febrile seizures plus (GEFS+) is an idiopathic generalized epileptic syndrome of heterogeneous phenotype. The cases described here are of two brothers, one with severe myoclonic epilepsy of infancy (Dravet syndrome) and the other myoclonic-astatic epilepsy. Their father experienced one simple febrile seizure in infancy and two generalized tonic-clonic seizures after head trauma in adulthood, and had generalized epileptiform activity in the electroencephalogram. He died in a severe sport accident before genetic testing could be performed. In both siblings, but not in their healthy mother, DNA analysis identified an unreported point mutation (c.3925 C>T) in exon 20 of the SCN1A gene. The missense mutation was therefore assumed to be inherited from the father, who had a very mild clinical picture, with a single febrile seizure and only occasional generalized tonic-clonic seizures. The offspring have GEFS+ phenotypes with opposite severity, an illustration of the broad intrafamilial variability of SCN1A gene mutations.