Hereditary fructose intolerance (HFI) is a carbohydrate metabolic disease of autosomal recessive inheritance. The basic deficit is deficiency of aldolase B, the enzyme catalyzing catabolism of fructose-1-phosphate, which is found only in intestinal mucosa, liver and kidney. Its main symptoms are abdominal pain, vomiting, hypoglycemia, and severe liver disease following the ingestion of fructose. Neurologic impairment is not typical in HFI, but it can occur in the acute phase of the disease. Neurologic impairment is related to the acute hepatic toxicity of fructose (hypoglycemia, abnormal coagulation, cardiovascular collapse). The 7 year-old German girl admitted because of generalized tonic clonic seizure. She had the first seizure at the age of 2, and was diagnosed as Lennox-Gastaut syndrome. Thereafter, frequent morning and midnight seizures were developed following indigestion of milk, sweety cake and cookies. Her family history was unknown because she was adopted from India at the 4 months of age. She showed developmental delay. After the ingestion of fructose, the patient experienced hypoglycemic episode within 60-90 minutes of the intake. Based on this finding, she was diagnosed as HFI. With fructose free diet, the patient became free of seizure even without the anticonvulsant, and improved in growth and development.