So as to clarify the conflicting data on the chromosome number of human endometrium, endometrial cells from six normal women and four patients with uterine bleeding were cultured in vitro in serial tissue culture, omitting trypsin, CO2 incubation and human serum. Based on chromosome studies of 5,301 dividing cells, including determinations of exact chromosome numbers of 2,242 metaphases and estimations of ploidy levels and complete karyotype analysis of 332 cells, normal human endometrium was shown to have the same chromosome number as other human tissues. Furthermore, the chromosomal constancy was the same as in normal skin tissue. Polyploidy incidence was higher in three patients with bleeding, and one of them had a higher degree of aneuploid variation. One patient with fibroma had only near-tetraploid cells in the sample of endometrium studied. A statistical analysis of aneuploid cells with 45 chromosomes suggested that the losses were nonrandom.