Hereditary fructose intolerance was diagnosed in a 69-year-old man on the basis of his medical history and the response to an intravenous fructose tolerance test. Three men of the same age as our patient were used as control subjects. Since the severity may vary and affected individuals self-impose fructose and sucrose restriction, they are essentially symptom free. The diagnosis can only be suspected by taking a careful dietary history. The prevalence of this condition in adults is unknown. It is rare but is likely to be more common than data in the literature would indicate.