Background/Aims: The gene responsible for familial adenomatous polyposis(FAP), called the APC gene, has been localized to chromosome 5q21 region, and many DNA probes that identify genetic polymorphism near this locus are now available. In this paper, we used DNA markers linked to APC locus to determine their usefulness in the presymptomatic diagnosis of 3 Korean FAP families. Methods: Three DNA probes(Ci lpl I, ECB27 and 69-III) were used to assess FAP inheritance by restriction fragment length polymorphism(RFLP) analysis. Results: The DNA probes Cl 1pl l and 69-III didn't show any polymorphism or informativeness in Korean FAP families, including the spouse population. The DNA probe ECB27 showed the Bgl IJ polymorph- ism with allele frequency of 0.77 and 0.23 in the spouse population. We could make a presympto- matic diagnosis in 7 out of l7 children (with the informativeness rate of 41%) by confirming whether they inherited the chromosome which has APC gene abnormality or not. However, this result does not guarantee 100% accuracy because recombination phenomenon can occur between the DNA marker and the APC gene. Conclusion: The DNA probes Cl 1pl I and 69-III are not helpful in presymptomatic diagnosis, but the DNA probe ECB27 can be used appropriately in presymptomatic diagnosis of FAP because it provides sufficient informativeness to the members of Korean FAP families. (Korean J Gastroenterol 1995;27:189 - 198)