• The effects of seizures on performance were investigated in a pair of identical twins who were concordant for incidence of epilepsy, but who had highly contrasting histories of seizure frequency. An evaluation of abilities focused on intelligence, academic achievement, neuropsychologic functions, and emotional and social adjustment. While both twins showed some disabilities, the twin with the larger number of seizures was less able in every area examined. The results suggest that the greater seizure frequency is related to the lesser abilities.