Most patients who present with the 18q- syndrome have an apparent terminal deletion of the long arm of chromosome 18. For precise phenotypic mapping of this syndrome, it is important to determine whether the deletions are terminal deletions or interstitial deletions. A human telomeric YAC clone has been identified that hybridizes specifically to the telomeric end of 18q. This clone was characterized and used to analyze seven patients with 18q deletions. By FISH and Southern blotting analysis, all patients were found to lack this chromosomal region on their deleted chromosome, demonstrating that the patients do not have cryptic interstitial deletions.