The effect of fructose on liver metabolism in patients with hereditary fructose intolerance (HFI) and in heterozygotes for HFI was studied by 31P magnetic resonance spectroscopy (31P-MRS). In patients with HFI (n = 5) ingestion of small amounts of fructose was followed by an increase in sugar phosphates and decrease in inorganic phosphate (Pi) in the liver that could be detected by 31P-MRS. 31P-MRS could be used to diagnose fructose intolerance and to monitor the patients' compliance with a fructose-restricted diet. In heterozygotes (n = 8) 50 g fructose given orally led to accumulation of sugar phosphates and depletion of Pi in the liver. Fructose also induced a larger increase in plasma urate in heterozygotes than in control subjects. The effect of fructose on liver Pi and plasma urate was most pronounced in heterozygotes with gout (n = 3). Heterozygosity for HFI may predispose to hyperuricaemia.