Genetic mosaicism" describes the presence of two or more populations of cells within a single individual that differ in their genomic constitution. Although the occurrence of asymmetric overgrowth in Wiedemann-Beckwith syndrome (WBS) suggests that mosaicism has some role in the WBS phenotype, no direct evidence for this has been published. WBS is a congenital overgrowth syndrome with variable phenotype linked to the imprinted gene cluster on chromosome region 11p15. We have performed a molecular survey of multiple organs and tissues in a case of WBS with a high degree of mosaic paternal 11p15 uniparental disomy (UPD). The organs most severely affected were those with the highest percentage of cells with UPD. In particular there was a striking difference in the degree of mosaicism for 11p15 UPD between the extremely enlarged left adrenal and non-enlarged right adrenal gland. This result indicates that the proportion of paternal 11p15 UPD cells correlates with the tissue phenotype of WBS. Our results suggest that high proportions of abnormal cells result from a combination of stochastic events and cell selection. Mosaicism may explain the variable phenotypes including hemihyperplasia and predisposition to childhood cancers in WBS patients.