Familial adenomatous polyposis represents approximately 1% of all colorectal cancers and is caused by germline mutations in the adenomatous polyposis coli (APC) gene. Most mutations are located within the first 2000 codons, and several mutational hot spots have been identified. The relative location of the mutation may be associated with the number of polyps and partially predicts specific phenotypic expression. Mutations associated with the attenuated phenotype are found predominantly in the 5' region of the gene or in the last third. We describe a patient with a mutation in codon 161 of the APC gene, which displays a phenotype most closely resembling the attenuated form of familial adenomatous polyposis, and review the literature, the implications of this mutation, and the importance of the molecular testing in the proper and more complete characterization of these patients. Differences in the APC mutation sites alone cannot completely account for intrafamilial and interfamilial variation in the polyposis phenotypes.