Autism and mental retardation (MR) show high rates of comorbidity and potentially share genetic risk factors. In this study, a rare∼ 2 Mb microdeletion involving chromosome band 15q13. 3 was detected in a multiplex autism family. This genomic loss lies between distal break points of the Prader–Willi/Angelman syndrome locus and was first described in association with MR and epilepsy. Together with recent studies that have also implicated this genomic imbalance in schizophrenia, our data indicate that this CNV shows considerable phenotypic variability. Further studies should aim to characterise the precise phenotypic range of this CNV and may lead to the discovery of genetic or environmental modifiers.