Ketogenic diet is a high-fat, low carbohydrate, low protein diet used in treatment of pediatric epilepsy since the 1920s. Currently it is used primarily to treat refractory childhood epilepsy. Few serious complications caused by ketogenic diet have been reported. Short-term complications include dehydration, hypoglycemia, vomiting, diarrhea, and refusal to eat. Long-term complications include kidney stones, recurrent infections, metabolic derangement, hypercholesterolemia, irritability, lethargy, and refusal to eat. We experienced 3 cases of infantile spasm patients, whose refractory seizures were controlled with ketogenic diet, but various urinary abnormalities developed.