Germline and somatic truncating mutations of the adenomatous polyposis coli gene (APC) are thought to initiate colorectal tumor formation in familial adenomatous polyposis syndrome and sporadic colorectal carcinogenesis, respectively. Recently, an isoleucine-lysine polymorphism at codon 1307 (I1307K) of the APC gene has been identified in 6-7% of the Ashkenazi Jewish population. To assess the risk of this common APC allelic variant in colorectal carcinogenesis, a cohort of unselected Turkish subjects with stomach or colorectal cancer (or both) was analyzed for the APC I1307K polymorphism. Genomic DNA was extracted from patients by obtaining all stomach and colon malign polipose tissues using nuclei lysis methods. Detection of the I1307K mutation was performed using the commercial Pronto APC kit according to the manufacturer's instructions. The APC I1307K allele was identified in 7 of 57 stomach carcinoma patients (12.3%; P > 0.05) and 30 of 56 colon carcinoma patients (53.6%; P < 0.05) using antigen-anticor interaction methods. Comparing the frequencies of the two separate population control groups, the APC I1307K allele is associated with an estimated relative risk of 1.9 for colorectal neoplasia. Furthermore, APC I1307K carriers had greater numbers of adenomas and colorectal cancers per patient than noncarriers. The conclusion is that the APC I1307K variant leads to increased adenoma formation and colorectal cancer. The estimated relative risk for carriers may justify specific clinical screening for Turkish people expected to harbor this allele, and genetic testing in the long term may significantly promote colorectal cancer prevention in this population.