We report on three siblings with mild mental retardation and minor dysmorphic features carrying an interstitial duplication of the long arm of chromosome 18 inherited from a healthy mosaic carrier mother. The duplicated region spanned between 18q21.31 and 18q22.2 for about 12 Mb. The distal duplications of 18q are rare and only a small number of subjects, manifesting quite different clinical outcomes, have been described. However, in most of these cases, molecular characterization was not available. We have reviewed nine patients, including three familial cases, displaying overlapping duplicated regions, and compared them with the present individuals in an attempt to delineate karyotype-phenotype correlation. © 2007 Wiley-Liss, Inc.