The authors report a 30-year-old identical twin who had prolonged febrile convulsions (FC) at the age of 8 months, left mesial temporal lobe epilepsy beginning at the age of 2 years, and left mesial temporal lobe sclerosis (MTS). The unaffected twin had short FC and meningitis at the age of 4 years but remained seizure free. Thus, prolonged FC in children younger than 4 years may precipitate later development of MTS.