Screening for molecular changes within the adenomatous polyposis coli (APC) gene, exons 11 14 and the 5' half of exon 15, encompassing the mutation cluster region within exon 15, was performed in 30 patients with Familial Polyposis Coli (FAP). All patients were studied by heteroduplex analysis (HA) and single strand conformation polymorphism (SSCP) and molecular changes were found in 7 cases. Protein truncation test (PTT) has been performed in 17 cases in which mutations have not been found earlier, and shortening of protein product was noted in 2 cases. In three cases common deletion of 5 bp at codon 1309 and in one 5 bp deletion at codon 1061 were found. In other cases the molecular changes were demonstrated as heteroduplexes in exon 14 (1 patient), in segments E and F (one patient each) of exon 15, and in two cases the heteroduplexes were within the overlapping sequences of segments E/F and F/G of exon 15, respectively. In families where the molecular changes were found by HA, 7 persons at high risk for FAP were found and advised to undergo regular endoscopic examinations. In three persons at risk the transfer of mutation was excluded.