Seven girls (age 5 to 10 years) with Rett syndrome were investigated extensively. In 6 patients elevations of blood pyruvate were found. Blood lactate levels were marginally elevated. Two patients had variably elevated blood glucose levels. Metabolic studies were otherwise normal apart from minimally elevated blood ammonia levels in 3 of 5 patients tested, 2 of whom were on valproic acid. All 7 patients had anticonvulsant resistant seizures. EEG changes included generalized slowing and multifocal spike wave discharges, and pseudo-periodic burst-suppression patterns during sleep. Respiratory monitoring revealed apneic episodes only during the waking record. Six patients were below the 5th centile for weight despite normal caloric intake. Treatment with ketogenic diets, using medium chain triglyceride (MCT) oil when possible, has improved seizure control in the 5 patients who could tolerate the diet. Slight behavioral and motor improvement has occurred in these 5 patients and 6 of 7 patients on high fat diets have gained weight. With a possible defect in carbohydrate metabolism and a difficult seizure disorder, use of a ketogenic diet is logical and appears to produce clinical benefit in patients with Rett Syndrome.