Germline mutations within the adenomatous polyposis coli (APC) gene, a tumor suppressor gene, are responsible for most cases of familial adenomatous polyposis (FAP), an autosomal dominantly inherited predisposition to colorectal cancer. To date, more than 300 germ-line causative mutations within this gene have been described (Beroud and Soussi, 1996). Of these, about 95% are chain-terminating mutations, and more than 60% have been localized within exon 15 (Nagase and Nakamura, 1993, Beroud and Soussi, 1996). Using polymerase chain reaction-single strand conformation polymorphism, protein truncation test (PTT) and DNA sequencing we have identified five new frameshift mutations (2523insCTTA, 2638delA, 2803insA, 3185delAA, 4145delTCATGT), all occurring within exon 15 and giving rise to truncated protein products. Two of these new mutations are of particular interest because of the unusual phenotypic features shown by probands. The phenotype of the proband bearing the 2523insCTTA mutation at codon 842 was very aggressive with onset of the symptoms at 12 years, while the patient bearing the 3185delAA mutation at codon 1062 exhibited features of an attenuated form of FAP (AAPC). Our data reiterate the great heterogeneity of the mutational spectrum in FAP that gives rise to an extreme variability of the clinical expression.