Prometaphase chromosome study of 12 persons with an established diagnosis of the Angelman syndrome demonstrated that 5 had a 15q12 deletion appearing similar to that commonly observed in the Prader-Willi syndrome. Phenotype-karyotype correlation did not show any obvious clinical differences between those with and those without the deletion and no clinical overlap between Angelman and Prader-Willi syndrome was apparent. Our survey suggests that 15q12 deletions are frequent in Angelman syndrome but presence of the deletion does not appear to distinguish different clinical phenotypes. Experience with the cytogenetic study of Prader-Willi syndrome predicts that considerable complexity will emerge between the presence of 15 chromosome abnormalities and clinical expression of Angelman syndrome.