For a subpopulation of drug-resistant epilepsies, a ketogenic diet constitutes the treatment of choice. A ketogenic diet is a high-fat, low-protein, and low-carbohydrate diet, which induces ketosis. Despite the use in treatment of epilepsy since 1924, the clinical efficacy was not demonstrated in a controlled, randomized trial until 2008, showing its capability of reducing seizure frequency with more than 50%. However, the exact mechanism of this form of treatment is still unknown. We report here a patient with drug-resistant epilepsy on a ketogenic diet, where a brain 18F-FDG PET examination demonstrated a severely decreased uptake in the cerebral cortex bilaterally.