The XY sex-reversal syndrome occurs when a phenotypic mure is born that has the karyotype of a stallion. The syndrome is manifested by both genotypic and phenotypic heterogeneity. The sex-reversed genetic condition occurs frequently within certain pedigrees where XY females have been found and can be readily detected by chromosome karyotyping. The phenotypic spectrum ranges from the feminine mare with a reproductive tract thai is within normal limits to the greatly masculinized mare. Pedigree analysis suggests that there arc two modes of inheritance: (I) an X-linked recessive or autosomal sex-limited dominant transmitted through the female and (2) an autosomal sex-limited dominant or a Y chromosomal mutation with variable expression transmitted through the male. The proportion of female to male progeny of the 69 top-producing Arabian stallions in the world is 53 to 47, indicating that the occurrence of genetic defects affecting the sex ratio such as the XY sex-reversal syndrome is relatively infrequent. In nine cases where the sex ratios of stallions deviated significantly from the expected sex ratio, the incidence of female infertility also increased. Cytogenetic screening of young animals would provide for early detection and avoid the expense and disappointment of continuous breeding attempts. Just as importantly, it would aid in the reduction and the possible elimination of this genetic condition from breeding herds.