We describe a girl who had been followed since birth for apparent Shprintzen-Goldberg syndrome (SGS), with macrosomia, long fingers and toes, and craniosynostosis, and presented at 4 years of age with bilateral Wilms tumors (also called nephroblastoma). Cytogenetic analysis of her peripheral blood revealed a de novo supernumerary marker chromosome. This stable marker chromosome is present in 19 of 20 lymphocytes analyzed, as well as in all 40 tumor cells (20 from each tumor) studied. Classical and molecular cytogenetic studies indicate that the marker is derived from an inverted duplication of chromosome 15q25.3 → qter and contains a neocentromere. The presence of this marker chromosome in our patient results in tetrasomy 15q25.3 → qter. The relationship between her genotype and phenotype are discussed in light of genes, including IGF1R and FES, mapped to the aneusomic segment. © 2002 Wiley-Liss, Inc.