The authors investigated 37 patients with established focal epilepsy in terms of the antiepileptic drug (AED) blood levels needed to achieve control or significant abatement of seizures; then they examined the patients by computerized axial tomography (CAT) and grouped them according as they did or did not show evidence of an organic food lesion of the brain. Statistical elaboration of the data revealed that CAT-positive patients required significantly higher mean AED blood levels for clinical effectiveness than did CAT-negative patients; also, single drugs failed to produce control at any plasma level, necessitating recourse to multidrug regimes, significantly more often in the CAT-positive than in the CAT-negative patient group. In the discussion the authors examine the possible reasons for the observed difference of therapeutic responses.