Long-term outcomes of the ketogenic diet in the treatment of epilepsy have not previously been reported. A retrospective chart review of children treated with the ketogenic diet for more than 6 years at the Johns Hopkins Hospital was performed. The response was documented at clinic visits and by telephone contacts; laboratory studies were obtained approximately every 6 to 12 months. Satisfaction and tolerability were assessed by means of a brief parental telephone questionnaire. In all, 28 patients (15 males, 13 females), currently aged 7 to 23 years, were identified. The median baseline seizure frequency per week at diet onset was 630 (range 1–1400). Diet duration ranged from 6 to 12 years; 19 remain on the diet currently. After 6 years or more, 24 children experienced a more than 90% decrease in seizures, and 22 parents reported satisfaction with the diet's efficacy. Ten children were at less than the 10th centile for height at diet initiation; this number increased to 23 at the most recent follow-up (p=0.001). Kidney stones occurred in seven children and skeletal fractures in six. After 6 years or more the mean cholesterol level was 201mg/dl, high-density lipoprotein was 54mg/dl, low-density lipoprotein was 129mg/dl, and triglycerides were 97mg/dl. Efficacy and overall tolerability for children are maintained after prolonged use of the ketogenic diet. However, side effects, such as slowed growth, kidney stones, and fractures, should be monitored closely.