We report on a 15-year-old girl with a previously undescribed de novo duplication of segment 4q13.1-->q22.2. The origin of the extrachromosomal material on 4q was unequivocally established by fluorescent in situ hybridization with a chromosome 4 painting probe. Clinical manifestations included moderate mental retardation, destructive behavior, and minor physical anomalies. An analysis of the literature on partial 4q trisomy led us to identify a region comprising bands 4q22-q23, which may be involved in the development of the acrorenal field.