Three individuals, ascertained for differing reasons, were found to have extra material in the proximal long arm of chromosome 15. The abnormal offspring of one of these also carried this chromosome. The extra material appears identical in all four individuals. The occurrence of this variant in patients of normal phenotype indicates that the region q11-q13 of chromosome 15 contains material which can be duplicated with little effect, unless the gene whose disruption causes Prader-Willi syndrome is involved at the breakpoint.