Background: Breastfeeding is known to have beneficial effects, but there is concern that breastfeeding during antiepileptic drug (AED) therapy may be harmful to cognitive development. Animal and human studies have demonstrated that some AEDs can adversely affect the immature brain. However, no investigation has examined effects of breastfeeding during AED therapy on subsequent cognitive abilities in children. Methods: The Neurodevelopmental Effects of Antiepileptic Drugs Study is an ongoing prospective multicenter observational investigation of long-term effects of in utero AED exposure on cognition. Between 1999 and 2004, we enrolled pregnant women with epilepsy who were taking a single AED (carbamazepine, lamotrigine, phenytoin, or valproate). We recently reported on differential AED effects on age 3 year cognitive outcomes. In this report, we focus on the effects of breastfeeding during AED therapy on age 3 cognitive outcomes in 199 children. Results: A total of 42% of children were breastfed. IQs for breastfed children did not differ from nonbreastfed children for all AEDs combined and for each of the 4 individual AED groups. Mean adjusted IQ scores (95% confidence intervals) across all AEDs were breastfed = 99 (96-103) and nonbreastfed = 98 (95-101). Power was 95% to detect a half SD IQ effect in the combined AED analysis, but was inadequate within groups. Conclusions: This preliminary analysis fails to demonstrate deleterious effects of breastfeeding during AED therapy on cognitive outcomes in children previously exposed in utero. However, caution is advised due to study limitations. Additional research is needed to confirm this observation and extend investigations to other AEDs and polytherapy. Copyright © 2010 by AAN Enterprises, Inc.