To investigate the molecular basis of hereditary fructose intolerance, we have studied 12 British patients, all of whom were found to carry a single mutation in the gene coding for aldolase B. We have estimated the frequency of this lesion, termed A149P, amongst affected individuals in the population and predict that a diagnosis may be made non-invasively in more than 83 per cent of cases by demonstrating the presence of this allele. Genetic diagnosis and detection of asymptomatic carriers of the disease may be achieved by the specific amplification of DNA derived from mouthwash samples followed by hybridization to allele-specific oligonucleotides.