The most common mutation in late-onset metachromatic leukodystrophy is a cytosine-to-thymine substitution in exon VIII. This mutation caused a substitution of leucine for proline at amino acid residue 426. We developed a rapid and simple method for the detection of 426Pro-->Leu mutation by polymerase chain reaction with mismatched primer. Although the 426Pro-->Leu mutation does not alter recognition sequence for restriction enzymes, we created a Pst I restriction site using a 3'-primer mismatched at one nucleotide. As a result, the mutation can be detected as a Pst I restriction fragment length polymorphism.