The idiopathic generalized epilepsies (IGE) comprise two major groups: the classical IGE and generalized epilepsy with febrile seizures plus (GEFS+). The classical IGE syndromes include childhood absence epilepsy; juvenile absence epilepsy; juvenile myoclonic epilepsy; and epilepsy with generalized tonic-clonic seizures alone. GEFS+ is a familial epilepsy syndrome, characterized by a spectrum of phenotypes. The phenotypes of GEFS+ include febrile seizures (FS), febrile seizures plus (FS+), FS/ FS+ and absences, myoclonic, atonic or partial seizures, myoclonic-astatic epilepsy and severe myoclonic epilepsy of infancy. Our study of 121 individuals in 20 families, where 84 had previously recognized GEFS+ phenotypes, expands the phenotypic spectrum of GEFS+ syndrome to include afebrile generalized tonic-clonic seizures with generalized spike wave or normal EEG in the absence of FS. To date, there are three ion channel genes (SCN1A, SCN1B and GABRG2) confi rmed as having a role in GEFS+, but none has been implicated in the majority of patients with GEFS+ phenotypes, such as those found in small families. Indeed it is likely that in most families, GEFS+ is a polygenic disorder resulting from the cumulative effect of a number of genes of lesser effect rather than the genes so far characterized in the few large families ascertained. Small GEFS+ families and bilineal inheritance in some add support for complex inheritance in a signifi cant proportion of families. The phenotypes of classical IGE occur in some GEFS+ families. The percentage of classical IGE phenotypes is 9% (11/121) of affected individuals in our study. This suggests that classical IGE phenotypes and GEFS+ phenotypes overlap in some GEFS+ families. Our study provides new insights into the inter-relationship of GEFS+ and classical IGE, where shared genetic determinants probably account for the overlap of these syndromes in some families.