The ketogenic diet is increasingly used for the management of difficult-to-control seizures in children. Here, we describe the first prospective study of the effects of the diet on development, behavior, and parenting stress. Participants were 65 children (36 males, 29 females) with intractable seizures, ages 18 months to 14 years 6 months, enrolled in a prospective study at the Johns Hopkins Hospital, Baltimore, MD, USA, to study the diet's efficacy. Children were assessed before diet initiation and at 1-year follow-up. At follow-up, 52% (34 of 65) children remained on the diet. Mean seizure frequency decreased from 25 per day before diet initiation to less than two per day 1 year later. At follow-up, mean developmental quotient showed statistically significant improvement (p<0.05), with significant behavioral improvements in attention and social functioning. Parental stress was essentially unchanged. No baseline factor examined predicted diet adherence, and the primary reason for diet discontinuation was insufficient seizure control. These preliminary results support prior anecdotal reports of the beneficial effects of the diet on cognition and behavior.