Retrospective insurance claims from the United States were analyzed to assess nonadherence to antiepileptic drugs (AEDs) and the association between AED nonadherence, seizures, and health care costs in elderly persons with epilepsy. Inclusion criteria were: age >= 65, epilepsy diagnosis between I January 2000 and 31 June 2006, >= 2 AED prescriptions, and insurance enrollment for >= 6 months pre- and >= 12 months post-AED initiation. Adherence was evaluated using the medication possession ratio (MPR), with MPR <0.8 defining nonadherence. Per-patient outcomes were evaluated over 12 months post-AED initiation. Of 1278 patients identified, 41% were nonadherent. Seizure, defined by epilepsy-related inpatient or emergency department admission, occurred in 12.1% of nonadherers versus 8.2% of adherers (P = 0.0212). Nonadherers had higher inpatient (+$872, P = 0.001), emergency department (+$143, P = 0.0008), other outpatient ancillary (+$1741, P = 0.0081), and total health care (+$2674, P = 0.0059) costs. AED adherence among elderly patients with epilepsy is suboptimal and associated with increased seizures and health care costs. (C) 2008 Elsevier Inc. All rights reserved.