Deletions, duplications, and rearrangements of the long arm of chromosome 15 are frequently associated with the clinical diagnosis of the Prader-Willi syndrome. However, a number of other clinical entities have also been associated with similar, if not identical, cytogenetic defects, arguing for clinical heterogeneity associated with abnormalities in this region of chromosome 15. We present 3 patients who all appear to have deletions in 15q11-15q12, such as described for many patients with Prader-Willi syndrome; however, none of these patients has classical clinical features of the Prader-Willi syndrome. The first patient is a child with Williams syndrome, the second, Angelman (Happy Puppet) syndrome, and the third is a child with hypotonia of infancy, obesity, and developmental delay, but who does not meet specific diagnostic criteria for the Prader-Willi syndrome. It is proposed that different molecular abnormalities involving specific points or segments along the long arm of chromosome 15 might account for the clinical diversity seen among these and other patients.