We report on a child with Jacobsen syndrome (JBS, OMIM 147791) and abnormalities consistent with Beckwith–Wiedemann syndrome (BWS, OMIM 130650). The constitutional karyotype was apparently normal, but FISH analysis with probes specific for the short and long arms of chromosome 11 found 11qter deletion with 11pter trisomy in 80% of the cells studied. Array-CGH identified breakpoints in the 11p15.3 and 11q24.1 regions consistent with Jacobsen and Beckwith–Wiedemann syndromes. We suggest that this chromosome imbalance results from a pericentric inversion of chromosome 11 inherited from the father, with mosaicism resulting from meiotic recombination of a paternal inversion followed by mitotic recombination during the first embryonic divisions. This hypothesis is supported by the results of microsatellite marker analysis. Three previous cases of pericentric inversion and recombination of chromosome 11 have been reported. Our case is unusual in that it combines the Jacobsen and Beckwith–Wiedemann syndromes with mosaicism. © 2013 Wiley Periodicals, Inc.