High-resolution karyotype analysis was performed on peripheral blood cultures from 26 patients with hereditary colorectal neoplasia. The aims of this study were: first, to determine the frequency of cytogenetically visible chromosome 5q deletions in familial adenomatous polyposis and, thus, whether routine karyotype analysis should be included in screening regimens for affected families; and, second, to search for chromosomal abnormalities in hereditary nonpolyposis colorectal cancer that might assist in localizing the gene or genes responsible. No cytogenetic abnormalities were detected among 21 unrelated patients with familial adenomatous polyposis and five with hereditary nonpolyposis colorectal cancer. We conclude that cytogenetic analysis is of no value in the management of families with typical familial adenomatous polyposis or Gardner's syndrome, and should be confined to those families with atypical features such as mental retardation or facial dysmorphism.