The most common pathology in adult patients with temporal lobe epilepsy is hippocampal sclerosis (HS), the etiology of which is not clear. There is a conflicting evidence in literature regarding whether HS is genetic or acquired. Twin studies can help to clarify the mechanisms of HS, but limited numbers of twins have been studied. We describe two monozygotic pairs, in whom the affected twin had mesial temporal lobe epilepsy and HS. The unaffected twin remained seizure free on long-term follow-up. HS was confirmed pathologically in one of the affected twins. Our data and other limited twin studies indicate that HS occurs as a consequence of prolonged repeated seizures or other events during childhood. In other words, some acquired factors may be more important than genetic ones in the etiology of HS (Fig. 2, Ref. 16). Full Text in PDF www.elis.sk.