BACKGROUND: Williams syndrome is characterized by supravalvular aortic stenosis, mental retardation and peculiar facial appearance. Its genetic etiology is considered to be hemizygotic deletion in Chromosome 7q11.23 which includes the elastin gene. We examined the deletion in Korean Williams syndrome patients and their parents. MATERIALS AND METHOD: Sixteen patients were selected through careful clinical examination including echocardiography and cardiac angiography. Hemizygotic deletion of elastin gene was determined in patients and 21 parents with fluorescent in situ hybridization (FISH) technique using the bacterial artificial chromosome clone 244H3 probe or commercial WSCR probe. RESULTS: FISH showed hemizygotic deletion of chromosome 7 in all sixteen patients but none of their parents showed deletion. CONCLUSION: Hemizygotic deletion of elastin gene can be determined by FISH with new probe 244H3 in clinically suspected Williams syndrome patients.