The ketogenic diet is carefully calculated by dietitians in an effort to achieve the child's ideal body weight, theoretically to improve seizure control. This study researched whether achieving a stable body mass index or ideal body mass index-for-age correlates with efficacy with the traditional ketogenic diet. The outcomes of 123 children started on the ketogenic diet were analyzed at clinic visits 3, 6, 9, and 12 months after diet onset. Children who were at 40% to 59% body mass index-for-age did not have higher efficacy than those at a higher or lower body mass index-for-age, except at the 12-month clinic visit (81% versus 48%; P = .02). No clear link was demonstrated between either an ideal body mass index or changes in the body mass index and seizure control in the management of children receiving a ketogenic diet. Attributing changes in seizure control to a rapid weight gain or loss may be unjustified.