We describe a new syndrome of familial temporal lobe epilepsy in 38 individuals from 13 unrelated white families. The disorder was first identified in 5 concordant monozygotic twin pairs as part of a large-scale twin study of epilepsy. When idiopathic partial epilepsy syndromes were excluded, the 5 pairs accounted for 23% of monozygotic pairs with partial epilepsies, and 38% of monozygotic pairs with partial epilepsy and no known etiology. Seizure onset for twin and nontwin subjects usually occurred during adolescence or early adult life. Seizure types were simple partial seizures with psychic or autonomic symptoms, infrequent complex partial seizures, and rare secondarily generalized seizures. Electroencephalograms revealed sparse focal temporal interictal epileptiform discharges in 22% of subjects. Magnetic resonance images appeared normal. Nine affected family members (24%) had not been diagnosed prior to the study. Pedigree analysis suggested autosomal dominant inheritance with age-dependent penetrance. The estimated segregation ratio was 0.3, indicating an overall penetrance of 60% assuming autosomal dominant inheritance. The mild and often subtle nature of the symptoms in some family members may account for lack of prior recognition of this common familial partial epilepsy. This disorder has similarities to the El mouse, a genetic model of temporal lobe epilepsy with a major gene on mouse chromosome 9, which is homologous with a region on human chromosome 3.