Williams syndrome is a rare neurodevelopmental disorder with variable phenotypic expression and a contiguous gene syndrome caused by deletion of the elastin gene. In our study, hemizygosity at the elastin locus was investigated using FISH analyses in 16 sporadic cases with a firm clinical diagnosis of Williams syndrome, and the characteristic features were evaluated. Fourteen patients were found to have deletions; 2 further patients did not have deletions of the elastin gene, but did have the clinical features. The presence of two copies of the elastin gene locus in a patient does not rule out Williams syndrome as a diagnosis. Since deletion of the elastin gene, which continues to be a useful confirmatory diagnostic test, cannot account for several features found in Williams syndrome, the non-deletion patients will be valuable in further delineation of the critical region responsible for the Williams syndrome phenotype.