Current knowledge on chromosomal mosaicism in human cell cultures is mostly based on cytogenetic banding methods. The recent development of high-resolution full-genome analysis methods applicable to single cells is providing new insights into genetic and cellular diversity. Here we study the genetic content of 92 individual human cells, including fibroblasts, amniocytes and embryonic stem cells (hESCs), using single-cell array-based comparative genomic hybridization (aCGH). We find that human somatic and embryonic stem cell cultures show significant fractions of cells carrying unique megabase-scale chromosomal abnormalities, forming genetic mosaics that could not have been detected by conventional cytogenetic methods. These findings are confirmed by studying seven clonal hESC sub-lines by aCGH. Furthermore, fluorescent in situ hybridisation reveals an increased instability of the subtelomeric regions in hESC as compared to somatic cells. This genetic heterogeneity may have an impact on experimental results and, in the case of hESC, on their potential clinical use.