Individuals who are homozygotes for mutations in DNA repair genes are at high risk for cancer. It is not well documented, however, if the heterozygous carriers of the mutation are also predisposed to cancer. To address the issue, xeroderma pigmentosum (XP) in Japan is an interesting candidate because of three major reasons: XP is an autosomal recessive disorder with an enormously elevated risk of skin cancer, the frequency of XP patients is higher in Japan than in other parts of the world, and more than half of Japanese XP patients are homozygous for the same founder mutation in the XPA gene. We screened archival blood samples from Japanese individuals who resided in Hiroshima or Nagasaki. A simple PCR-RFLP method was developed that is highly specific for detection of XPA heterozygotes carrying the founder mutation. We identified nine XPA heterozygotes among 1,020 individuals screened for a prevalence of 0.88%. This rate, if representative, implies that there are about 1 million carriers of the XPA founder mutation in the Japanese population. Thus, investigation of their cancer risk may be warranted.