In the course of presymptomatic diagnosis in families with familial adenomatous polyposis (FAP) we screened 202 unrelated patients for mutations in the APC gene. Germ-line mutations were identified in 20.8% of the index patients by a single step screening procedure based on heteroduplex analysis of a PCR product encompassing codons 1027-1384 of the APC gene. The most common mutations in our sample were a 5 bp deletion at codon 1309 in 9% of the families, a 5 bp deletion at codon 1061 in 5% and a 4 bp deletion at codon 1068 in 2.5% of the families. In addition, 11 novel mutations localized within the exons 11-15 of the APC gene were identified by the heteroduplex or SSCP methods.