Purpose: To determine whether changes in the high-energy phosphates occur with use of the ketogenic diet in patients with intractable epilepsy. Methods: 31P magnetic resonance spectroscopic imaging studies were performed at 4.1 T in seven patients with intractable epilepsy (four Lennox- Gastaut syndrome, one absence, one primary generalized tonic-clonic, and one partial complex) before and after institution of the ketogenic diet. Coronal 1H anatomic imaging also was performed to provide correlation to the 31P data. Results: Taking the patients as a group, the ratio of phosphocreatine (PC)/γ-adenosine triphosphate (ATP) measured at baseline (regular diet) compared with that measured after the ketogenic diet showed a small but significant increase from 0.61 ± 0.08 to 0.69 ± 0.08 (p < 0.05). Comparing the ratio of PCr inorganic phosphorus (P(i)) measured at baseline with the postketogenic diet, there was a significant increase from 2.45 ± 0.27 to 2.99 ± 0.44 (p < 0.05). Conclusions: As a group, improvement of energy metabolism occurs with use of the ketogenic diet. This is in agreement with the chronic ketosis studies performed earlier in rodents.