The clinical and genetic relationships of febrile seizures and the generalized epilepsies are poorly understood. We ascertained a family with genealogical information in 2000 individuals where there was an unusual concentration of individuals with febrile seizures and generalized epilepsy in one part of the pedigree. We first clarified complex consanguineous relationships in earlier generations and then systematically studied the epilepsy phenotypes in affected individuals. In one branch (core family) 25 individuals over four generations were affected. The commonest phenotype, denoted as 'febrile seizures plus' (FS+), comprised childhood onset (median 1 year) of multiple febrile seizures, but unlike the typical febrile convulsion syndrome, attacks with fever continued beyond 6 years, or afebrile seizures occurred. Seizures usually ceased by mid childhood (median 11 years). Other phenotypes included FS+ and absences, FS+ and myoclonic seizures, FS+ and atonic seizures, and the most severely affected individual had myoclonic-astatic epilepsy (MAE). The pattern of inheritance was autosomal dominant. The large variation in generalized epilepsy phenotypes was not explained by acquired factors. Analysis of this large family and critical review of the literature led to the concept of a genetic epilepsy syndrome termed generalized epilepsy with febrile seizures plus (GEFS+). GEFS+ has a spectrum of phenotypes including febrile seizures, FS+ and the less common MAE. Recognition of GEFS+ explains the epilepsy phenotypes of previously poorly understood benign childhood generalized epilepsies. In individual patients the inherited nature of GEFS+ may be overlooked. Molecular genetic study of such large families should allow identification of genes relevant to febrile seizures and generalized epilepsies.