We report on a boy with three cell lines: 46,XY, r(11)(p15.5,q25)/45,XY,-11 /47,XY, r(11)(p15.5,q25)x2, with minor anomalies and mental retardation who developed asynchronous bilateral Wilms tumors (WTs). Array comparative genomic hybridization (CGH) performed on peripheral blood leukocytes of the patient led to the identification of a constitutional duplication of 4.8 Mb at 11p15.5–11p15.4. This duplication was found to involve the chromosome of paternal origin, and occurred in tandem on the ring chromosome 11. Despite the constitutive duplication of the paternal 11p15 chromosome region, the patient showed no sign of Beckwith-Wiedemann syndrome. However, the molecular characterization of the two neoplasias was consistent with their independent origin and showed that they arose from the two distinct cellular clones with the ring chromosome, indicating that this anomaly is likely to have caused the patient's susceptibility to WT development. © 2010 Wiley-Liss, Inc.