Isoleucine (Ile)-valine (Val) polymorphism, which is caused by a point mutation from A to G in exon 7, is reported to be associated with an elevated risk of lung cancer among Japanese Because CYP 1 Al catalyzes bioactivation of environmental procarcinogens, such as benzolalpyrene, it is very important to study the clinical meaning of Ile-Val polymorphism using an epidemiological study In an epidemiological study, easy, economical, rapid and reliable identification of the CYP 1 A 1 genotype is necessary The present study shows that the new method, designed restriction fragment length polymorphism (designed RFLP), can detect Ile-Val polymorphism of CY Pl A 1 The Ile-Val polymorphism detected using this new method was consistent with that found by the allelespecific PCR amplifications (ASA) method in six cases tested This new method detected Ile-Val polymorphism of CY Pl AI using 240 healthy Japanese who lived in the northern Kyusyu region The frequency of the genotypes was as follows: Ile/Ile, 159 ( 66 2 %); Ile/Val, 65 ( 27 1 %); Val/Val, 16 ( 6 7 %) The frequency of the Ile gene was 0.798 and that of the Val gene, 0 202 There was no difference in Ile-Val polymorphism based on sex or age. Racial differences influenced the distribution of this polymorphism, but Japanese regional differences did not Since this new method, designed RFLP, is rapid, reliable and suitable for large-scale screening of polymorphisms, it may be used routinely to detect Ile-Val polymorphism of