In relation to the general problem of neuropsychological conditions in epileptic and migraine patients, the literature is reviewed and new data are presented emphasizing the specific heuristic value of methods investigating interhemispheric functional balance. Available data indicate that: a) in epileptic monohemispheric non-lesional patients an interhemispheric functional change in favour of the epileptic hemisphere is detectable, more manifest when the left hemisphere is involved; b) in migraine patients interhemispheric functional changes are detectable only during the paroxysmal phase, more manifest when the left hemisphere is involved.