The authors analyzed blood metabolites in nine children with epilepsy prior to starting the ketogenic diet (KD) and 3 to 4 weeks after KD therapy. Elevated beta-hydroxybutyrate and cortisol levels were observed in all children on the KD. Free fatty acids increased 2.2-fold on the KD, with significant elevations in most polyunsaturated fatty acids (PUFA; arachidonate increased 1.6- to 2.9-fold and docosahexaenoate increased 1.5- to 4.0-fold). The rise in total serum arachidonate correlated with improved seizure control. Elevated PUFA may represent a key anticonvulsant mechanism of the KD.