The association between febrile convulsions and temporal lobe epilepsy is recognized, but is controversial. We attempted to clarify this and to determine which attributes of febrile convulsions were associated with temporal lobe epilepsy. During a study of genetic linkage in febrile convulsions, families with at least four affected members were identified. Clinical features of febrile seizures and EEG findings were compared in those who did and did not develop later afebrile seizures. In six selected families, 59 family members had febrile convulsions. Temporal lobe epilepsy developed in eight of these, whereas only one of 213 family members with no febrile convulsions had temporal lobe epilepsy (P < 0.0001). The mean durations of febrile convulsions in those with and without progression to temporal lobe epilepsy were 100 and 9 min, respectively (P = 0.02). Five patients had temporal lobectomies, which revealed mesial temporal sclerosis in all cases. Four patients who developed other types of epilepsy had a mean duration of febrile convulsions of 90 min. The total number, the maximum number in any one day, and the age at onset of febrile convulsions did not differ significantly between groups. Only two patients had neurological deficits, both of whom subsequently had nonlocalizable partial epilepsy. In these families, selected to reduce genetic and phenotypic heterogeneity, a strong association was evident between febrile convulsions and temporal lobe epilepsy with mesial temporal sclerosis. A prolonged febrile convulsion was the most important determinant of this association.