Hereditary fructose intolerance (HFI) is a rare metabolic disease affecting fructose metabolism. After ingestion of fructose, patients may present with clinical symptoms varying from indefinite gastrointestinal symptoms to life-threatening hypoglycaemia and hepatic failure.A 13-year-old boy was referred to the department of metabolic diseases because of an abnormal fructose loading test. He was known with persistent gastrointestinal symptoms since infancy. His dietary history revealed an avoidance of fruit and sweets. Because malabsorption was suspected, an oral fructose loading test was performed. During this test, he developed severe vagal symptoms which were probably caused by a potentially fatal hypoglycaemia. The diagnosis of HFI was confirmed by genetic analysis.A good dietary history may be of important help in the diagnosis of HFI. On suspicion of HFI, genetic analysis is easy and the first choice in the diagnostic work-up. With timely diagnosis and adequate dietary treatment patients have an excellent prognosis. Fructose loading tests as part of the diagnostics can be dangerous.