In Japan 1052 FAP patients in 688 family trees have been registered at the National Polyposis Center. In Hamamatsu University School of Medicine, we have been treating 50 patients in 28 family trees. Among these families, we analysed the site of mutations of the APC gene, which so far has been clarified in 21 family trees. Presymptomatic diagnosis was possible in 12 individuals among the carriers in 6 FAP families. Most of the mutations were clustered within the 5'half of exon 15. There was no correlation between extra colonic phenotypic expression and site of mutation, however, the desmoid should be studied further, as the number of cases was limited.