Angelman and Prader–Willi syndromes are reciprocal imprinting disorders caused by loss of maternally or paternally expressed genes, respectively, within 15q11.2–q13. Angelman syndrome (AS; OMIM 105830) is a neurodevelopmental disorder and is due to the loss of maternally expressed UBE3A gene. Prader–Willi syndrome (PWS; OMIM 176270) is a clinically distinct disorder caused by the loss of paternally expressed genes in the human chromosome region 15q11.2–q13. Recently published data strongly suggest a role for the paternally expressed small nucleolar RNA (snoRNA) cluster, SNORD116, in PWS etiology. Uniparental disomy (UPD) 15 is one of the important causes of PWS and AS. Interestingly, balanced and unbalanced chromosomal aberrations in the form of Robertsonian translocation, isochromosomes, supernumerary marker chromosomes and copy number variations have been strongly linked with the occurrence of UPD. Here we report on a very unique case with a mosaic isochromosome for the entire long arm of 15, that is, i(15)(q10), resulting in mosaic uniparental isodisomy for 15q and with no copy number alterations. This is the first report of UPD15 constituted by a mosaic, but copy number neutral chromosomal rearrangement in a patient with a variant PWS-like phenotype. © 2013 Wiley Periodicals, Inc.