Although mouse embryonic stem cell lines (mESCs) have been established since 1981, systematic studies about chromosomal changes during culture are lacking. In this study, we report the results of a cytogenetic analysis performed on three mESC lines (named UPV02, UPV06 and UPV08) cultured for a period of 3 months. At time intervals, the variation of the chromosome number together with the expression of markers of the undifferentiated status, i.e., OCT-4, SSEA-1, FOM-1 and alkaline phosphatase activity, were determined. The three mESC lines showed a progressive loss of euploid metaphases during the 3 months period of culture. Chromosome abnormalities were accumulated at the latest passages analysed. Metacentric chromosomes were the most frequent chromosome abnormality observed throughout the period of culture. Interestingly, in coincidence with, or few passages after, the drop of euploidy, the alkaline phosphatase activity was partially or totally lost, whereas the OCT-4, SSEA-1 and FOM-1 stem markers were always positive throughout the period of culture. Our results remark the necessity to perform the karyotype analysis during culture in order to develop new culture conditions to maintain the correct chromosome complement in long-term culture of mESC lines.