The problem of the functioning specificity of sex chromosomes during the early stages of embryogenesis in man and the associated problem of the sex ratio in spontaneous and induced abortions, as well as in newborns, remains open. We have conducted a cytogenetic examination of 342 spontaneous abortions divided into three clinical groups on the basis of the severity of the developmental disturbances of the embryo: spontaneous abortions sensu stricto with a developed embryo without any significant intrauterine delay of development (n = 100), nondeveloping pregnancies (n = 176), and anembryonic fetuses (n = 66). The frequency of chromosomal mutations in these groups was 22.0, 48.3, and 48.5%, respectively. Statistical analysis has demonstrated significant differences between the studied groups in the frequencies of the normal and abnormal karyotypes: the major contributions to these differences were associated with autosomal trisomy, triploidy, and 46,XY karyotype. The presence of 46,XY may reflect specific genetic mechanisms of prenatal mortality of embryos with normal karyotype, associated with sex and/or with the imprinting of X-chromosomes. The sex ratio in spontaneous abortions with normal karyotype was as follows: 0.77 for spontaneous abortions with well-developed embryos without any significant intrauterine delay of development; 0.60 for non-developing pregnancies; and 0.31 for anembryonic fetuses. An analysis of DNA from the embryos and their parents has demonstrated a low probability of contamination of cell cultures with mother cells as a possible source of prevalence of embryos with 46,XX karyotype among spontaneous abortions. Nondeveloping pregnancies and anembryonic fetuses showed statistically significant differences in the sex ratio (1.11) from the control group consisting of medical abortions. Differences in the sex ratio were due to an increasingly lower proportion of embryos with karyotype 46,XY (relative to the expected one) among the fetuses with an increased severity of developmental disturbances. The statistical "chances ratio" index also provided evidence that embryos with 46,XY karyotype had a higher propensity to produce a well-formed fetus as compared with the female embryos. We propose that the expression of genes of the maternal X-chromosome in XY embryos supports a more stable development during early embryogenesis as compared with XX embryos. In the latter case, normal development is coupled with the operation of an additional mechanism for compensation of the dose of X-linked genes. Operation of this mechanism increases the probability of disturbances in female embryos. A higher viability of XY embryos during the early stages of ontogenesis in man appears to explain their underrepresentation in samples of spontaneously aborted embryos and appears to be the major factor responsible for the deviation of the sex ratio from the theoretically expected value.