Objectives: To investigate the significance of “subtherapeutic” vs “therapeutic” antiepileptic drug (AED) plasma levels with respect to treatment adherence. Material and methods: One hundred and seventy patients with refractory temporal lobe epilepsy who underwent video-EEG monitoring in view of a surgical indication had their AEDs (carbamazepine, phenobarbital, phenytoin, and valproate) rapidly withdrawn following a standardized schedule. Plasma levels were measured at admission, and during the 2 days of drug withdrawal. Adherence and nonadherence were identified by the development of plasma levels from day 1 through day 3. Frequencies of an initial level below the reference range in both groups were compared. Results: Adherence was found in 73.2% of cases, and nonadherence in 26.8%. Low levels were seen equally often (about 1/4 of cases) in adherent and nonadherent cases. The vast majority (73.7%) of low levels had another explanation than nonadherence (eg low-dose treatment or enzyme induction). Of 42 nonadherent cases, the vast majority of 76.2% had unsuspicious plasma levels at admission. Conclusions: “Subtherapeutic” AED plasma levels only rarely are caused by nonadherence whereas levels in the “therapeutic range” by no means prove that the patient is adherent to treatment. For meaningful interpretation, any level needs to be compared with other levels of the same patient. Our findings strongly emphasize the principle of individualized therapeutic AED monitoring as promoted by the Therapeutic Strategies Commission of the ILAE.