The Williams–Beuren syndrome (WBS) locus, at 7q11.23, is prone to recurrent chromosomal rearrangements, including the microdeletion that causes WBS, a multisystem condition with characteristic cardiovascular, cognitive, and behavioral features. It is hypothesized that reciprocal duplications of the WBS interval should also occur, and here we present such a case description. The most striking phenotype was a severe delay in expressive speech, in contrast to the normal articulation and fluent expressive language observed in persons with WBS. Our results suggest that specific genes at 7q11.23 are exquisitely sensitive to dosage alterations that can influence human language and visuospatial capabilities.