Objective: To examine the effect of maternal folic acid supplementation and maternal plasma folate and antiepileptic drug (AED) concentrations on language delay in AED exposed children of mothers with epilepsy. Methods: Children of mothers with and without epilepsy enrolled 1999-2008 in the Norwegian Mother and Child Cohort study were included. Information on medical history, AED use and folic acid supplementation during pregnancy was collected from parent-completed questionnaires. Maternal plasma folate and maternal plasma and umbilical cord AED concentrations were measured in blood samples from gestational week 17-19 and immediately after birth, respectively. Language development at 18 and 36 months was evaluated by the Ages and Stages Questionnaires. Results: A total of 335 AED exposed children of mothers with epilepsy and 104,222 children of mothers without epilepsy were surveyed. For those with no maternal periconceptional folic acid supplementation, the fully adjusted OR for language delay in AED exposed children compared to the controls at 18 months was 3.9 (CI 1.9-7.8, p<0.001) and at 36 months 4.7 (CI 2.0-10.6, p<0.001). When folic supplementation was used, the corresponding ORs for language delay were 1.7 (CI 1.2-2.6, p=0.01) and 1.7 (CI 0.9-3.2, p=0.13), respectively. The positive effect of folic acid supplement use on language delay in AED exposed children was only significant when supplement was used in the period from four weeks before the pregnancy and until the end of the first trimester. Conclusion: Folic acid use early in pregnancy may have a preventative effect on language delay associated with in utero AED exposure.