Germ-line mutations in the adenomatous polyposis coli (APC) gene result in familial adenomatous polyposis coli (APC), an inherited syndrome that predisposes affected individuals to early onset of colorectal cancer. Somatic APC mutations also have been detected in sporadic colon tumors. We have used single strand conformational polymorphism (SSCP) analysis to scan a region of the APC gene that frequently is mutated in both APC and sporadic colorectal cancer. Four truncating mutations were found between codons 1060 and 1327 in 17 of 68 unrelated APC individuals. Fourteen of these persons carried either of two previously described five-nucleotide deletions which represent about 20% of APC mutations in these pedigrees. Patients with mutations in this region of exon 15 develop a classic APC colonic phenotype with multiple, diffuse adenomas developing by the second or third decade. However, the density of adenomas and the extracolonic disease manifestations associated with this syndrome are variable among individuals with identical APC mutations.