PURPOSE: Delayed diagnosis of early onset epilepsy is a potentially important and avoidable complication in epilepsy care. We examined the frequency of diagnostic delays in young children with newly presenting epilepsy, their developmental impact, and reasons for delays. METHODS: Children who developed epilepsy before their third birthday were identified in a prospective community-based cohort. An interval >/=1 month from second seizure to diagnosis was considered a delay. Testing of development at baseline and for up to 3 years after and of intelligence quotient (IQ) 8-9 years later was performed. Detailed parental baseline interview accounts and medical records were reviewed to identify potential reasons for delays. Factors associated with delays included the parent, child, pediatrician, neurologist, and scheduling. RESULTS: Diagnostic delays occurred in 70 (41%) of 172 children. Delays occurred less often if children had received medical attention for the first seizure (p < 0.0001), previously had neonatal or febrile seizures (p = 0.02), had only convulsions before diagnosis (p = 0.005), or had a college-educated parent (p = 0.01). A >/=1 month diagnostic delay was associated with an average 7.4 point drop (p = 0.02) in the Vineland Scales of Adaptive Behavior motor score. The effect was present at diagnosis, persisted for at least 3 years, and was also apparent in IQ scores 8-9 years later, which were lower in association with a diagnostic delay by 8.4 points (p = 0.06) for processing speed up to 14.5 points (p = 0.004) for full scale IQ, after adjustment for parental education and other epilepsy-related clinical factors. Factors associated with delayed diagnosis included parents not recognizing events as seizures (N = 47), pediatricians missing or deferring diagnosis (N = 15), neurologists deferring diagnosis (N = 7), and scheduling problems (N = 11). SIGNIFICANCE: Diagnostic delays occur in many young children with epilepsy. They are associated with substantial decrements in development and IQ later in childhood. Several factors influence diagnostic delays and may represent opportunities for intervention and improved care.