Mosaicism involving a normal cell line and an unbalanced autosomal translocation are rare. In this study we present three new cases with such a mosaicism, which were detected by Single Nucleotide Polymorphism (SNP) array analysis in our routine diagnostic setting. These cases were further characterized using Fluorescence in situ Hybridisation (FISH) analysis and conventional karyotyping.The first case is a mentally retarded male who carries an unbalanced translocation in 87% of his cells. The phenotypically normal mother carries the balanced form of the translocation in all her cells. The second case is a phenotypically normal female who has an unbalanced translocation in 52% of her cells. The inheritance could not be determined. The third case is a female referred for Rubinstein-Taybi syndrome who carries an unbalanced translocation in 60% of her cells. Both parents of this case showed a normal karyotype.The mechanisms that might be responsible for these mosaic karyotypes are discussed. Furthermore, we demonstrate that high-resolution whole-genome SNP array is a powerful tool to reveal cryptic unbalanced translocations and mosaicisms, including the more rare cases.