Hereditary fructose intolerance (HFI) is a recessive genetic disorder with an estimated disease frequency of 1 in 20,000 and a carrier frequency of 1 in 70. Affected individuals are unable to assimilate fructose from fruit sugars and may develop severe hypoglycemia, metabolic problems, and death if misdiagnosed. Those who survive childhood learn to avoid sweets, effectively preventing further symptoms and complications. The disease is caused by a genetically defective hepatic enzyme, aldolase B. Traditionally, diagnosis has been made by intravenous fructose challenge or by liver biopsy, both difficult and risky invasive tests. Identification of mutations of the aldolase B gene by analysis of DNA from blood leukocytes is now possible, allowing for potential noninvasive diagnosis of subjects at risk in the future. The authors demonstrate heterozygosity for an aldolase B gene mutation in a patient with HFI.