The classical ketogenic diet, the medium chain triglyceride (MCT) diet, and a modified MCT diet were used in the treatment of 55 children and 4 adults with intractable epilepsy at the University and Clinical Departments of Paediatrics, John Radcliffe Hospital, Oxford, England. The main types of seizures were drop attacks (24), infantile spasms (7), tonic-clonic (6), partial complex (5), primary absence (2), myoclonic absence (2), complex absence (4), and partial simple (4). Forty-five of the patients were under ten years of age. Cooperation and compliance were good and 57 patients completed at least six weeks dietary management and some continued for periods up to four years. Fifty-six had significantly elevated blood ketones bodies and all were reported to have ketonuria. Eighty-one percent showed greater than 50% reduction in seizure frequency. The response was independent of the type of diet used; all three diets appeared to be equally effective in children under the age of 15 years. The high fat diet was found to be palatable by all of the children but the adults found the restrictions unacceptable. Large quantities of MCT oil were also found to be unpalatable by all age groups. No patient found the cream and butter content of the classical diet to be unacceptable. Analysis of variance failed to show any significant difference between the type of seizure and the success of treatment. The EEG showed improvement in 14 cases, 9 while the children were on the MCT diet and 5 while on the classical diet. Nausea, vomiting or abdominal discomfort occurred in approximately one-half the patients taking the MCT diet; drowsiness occurred in 25% at the introduction of the diet. Loss of ketosis occurred during the prodromal phase of an intercurrent illness and was often accompanied by an increase in the frequency of seizures. Two children under one year of age showed no increase in weight, length or head circumference during a six month period on the diet.