The molecular basis of hereditary fructose intolerance (HFI) was studied in 50 subjects (41 pedigrees, 82 apparently independent mutant alleles of aldolase B) by direct analysis of aldolase B genes amplified by means of the polymerase chain reaction. The mutation A149P (ala 149----pro) was found in 67% of alleles but was significantly more common in patients from northern than from southern Europe. Two other point mutations of aldolase B were identified. A174D (C----A; ala 174----asp) was found in subjects from Italy, Switzerland, and Yugoslavia (overall frequency 16%) but not in those from the United Kingdom, France, or the United States. L288 delta C carried a single base-pair deletion causing frameshift at codon 288 and was restricted to Sicilian subjects. By testing for these mutations in amplified DNA with a limited panel of allele-specific oligonucleotides, more than 95% of HFI patients will be susceptible to genetic diagnosis.