ObjectiveTo estimate the treatment gap between a new epilepsy diagnosis and antiepileptic drug (AED) initiation in the United States.MethodsRetrospective claims-based cohort study using Truven Health MarketScan databases (commercial and supplemental Medicare, calendar years 2010–2015; Medicaid, 2010–2014) and a validation study using PharMetrics Plus Database linked to LRx claims database (2009–2014). Persons met epilepsy diagnostic criteria, had an index date (first epilepsy diagnosis) with a preceding 2-year baseline (1 year for persons aged 1 to <2 years; none for persons <1 year), and continuous medical and pharmacy enrollment without epilepsy/seizure diagnosis or AED prescription during baseline. Outcomes included percentage of untreated persons (no AED prescription) up to 3 years' follow-up and comparative outcomes (incidence rate ratio: untreated persons/treated persons), including medical events and health care resource utilization.ResultsIn the primary study, 59,970 persons met selection (or inclusion) criteria; 36.7% of persons with newly diagnosed epilepsy remained untreated up to 3 years after diagnosis. In the validation study (N = 30,890), 31.8% of persons remained untreated up to 3 years after diagnosis. Lack of AED treatment was associated with an adjusted incidence rate ratio (95% confidence interval) of 1.2 (1.2–1.3) for medical events, 2.3 (2.2–2.3) for hospitalizations, and 2.8 (2.7–2.9) for emergency department visits.ConclusionsOne-third of newly diagnosed persons remain untreated up to 3 years after epilepsy diagnosis. The increased risk of medical events and health care utilization highlights the consequences of delayed treatment after epilepsy diagnosis, which might be preventable.