Cytogenetic abnormalities in the Prader-Willi/Angelman syndrome (PWS/AS) critical region have been described in individuals with autism. Maternal duplications and linkage disequilibrium in families with autism suggest the existence of a susceptibility locus at 15q11–q13. Here, we describe a 6-year-old girl diagnosed with autism, developmental delay, and delayed expressive and receptive language. The karyotype was designated de novo 47, XX, idic(15)(q13). Fluorescence in situ hybridization (FISH) and molecular analysis with 15q11–q13 markers revealed an additional copy of the region being of maternal origin. Duplication of the 15q11–q13 segment represents the most consistent known chromosomal abnormality reported in association with autism. This present case report reinforces the hypothesis that additional copies of this chromosome segment are causally related to autism.