A dysmorphic boy with severe mental retardation was found on array CGH to have an insertional translocation of chromosome 16p13.3 into the short arm of chromosome 22, karyotype 46,XY,.ish der(22),ins(22;16)(p13;p13.3p13.3) de novo. His clinical features overlap with the reported cases of 'duplication 16p' syndrome, namely a round face, hypertelorism, a long philtrum, micrognathia, a thin upper lip, a posterior cleft palate and low set, simple ears, clubbed feet, severe developmental delay, psychomotor retardation and seizures. This 4-year boy with trisomy 16p13.3 has the smallest duplication reported of this critical region, which could not be detected without array CGH. The maximal duplicated region is gene rich and contains about 80 genes and/or candidate genes. Assignment of the genes that contribute to the observed phenotype awaits the characterisation of other patients with small duplications in this region.