Williams-Beuren syndrome (WBS) is a rare neurodevelopmental disorder, characterized by distinct facial changes, growth deficiency, mental retardation, supravalvular aortic stenosis (SVAS)/peripheral pulmonary stenosis, and associated at times with infantile hypercalcemia. A pilot study has been carried out to assess the reliability of the detection of hemizygosity at the elastin locus by fluorescence in situ hybridization (FISH) analysis as a diagnostic test in both classical and atypical WBS. Eight subjects with classical WBS and four others in whom a diagnosis could not be confirmed on clinical criteria alone were enrolled. In the classical WBS group, five (5/8) had a visible interstitial 7q11.22-11.23 deletion detected by high-resolution banding, and all (8/8) had a submicroscopic deletion of the elastin locus on chromosome 7 by FISH analysis. In the atypical WBS group, only one (1/4) had elastin deletion. The other three, with isolated SVAS, had normal development and minimal signs of WBS. Furthermore, the patients with microscopic 7q11.22-11.23 deletion have more associated features of WBS than those without visible interstitial deletions by high-resolution banding. These results, therefore, emphasize the importance of a combined high-resolution and molecular cytogenetic (i.e., FISH) approach to diagnosis and suggest that the degree to which microscopic/submicroscopic deletions of chromosome 7 extending in beyond the elastin locus may explain some of the phenotypical variability found in WBS.