The ketogenic diet, originally introduced in the 1920s, has been undergoing a recent resurgence as an adjunctive treatment for refractory epilepsy, particularly in children. In this difficult-to-treat population, the diet exhibits remarkable efficacy with two-thirds showing significant reduction in seizure frequency and one-third becoming nearly seizure-free. There are several reasons to suspect that the ketogenic diet may also have utility as a mood stabilizer in bipolar illness. These include the observation that several anticonvulsant interventions may improve outcome in mood disorders. Furthermore, beneficial changes in brain-energy profile are noted in subjects on the ketogenic diet. This is important since global cerebral hypometabolism is a characteristic of the brains of depressed or manic individuals. Finally, the extracellular changes that occur in ketosis would be expected to decrease intracellular sodium concentrations, a common property of all effective mood stabilizers. Trials of the ketogenic diet in relapse prevention of bipolar mood episodes are warranted.