Familial adenomatous polyposis (FAP), a dominantly inherited disease, is caused by a mutation in the adenomatous polyposis coli gene in chromosome 5q21. The gene has 15 exons, a physical length of 10 Kb and an open reading frame of 8.5 Kb. Exon 15 codes 66% of the mRNA and has a mutation cluster region which accounts for over 50% of mutations. The disease usually leads to the appearance of hundreds of adenomatous polyps in the transverse and descending colon between puberty and age 20 years and to colon cancer before the age of 40. Early detection is essential to prevent the development of metastasizing cancer. Since 1994 we have recruited 23 families for genetic counseling. DNA was obtained from 19 unrelated FAP patients and 219 high risk relatives in 19 unrelated families following confirmation of the diagnosis. In addition to linkage studies, direct mutational analysis was performed using the protein truncation test for most of exon 15 and single strand conformation polymorphism analysis for the other exons. These exons account for most of the mutations identified to date. Of 19 unrelated probands, 14 had detectable mutations. Exon 15 accounted for 6 families, exons 5, 7 and 14 for 1 each, exon 9 for 3, and exon 8 for 2. Combined mutational and linkage analysis identified 18 presymptomatic carriers who received genetic and clinical counseling. Our FAP patients did not differ significantly from those of larger studies in other countries with regard to the distribution of the mutations, gender and genotype-phenotype correlation, or ethnic distribution.