Analysis of mitotic chromosomes revealed the consistent lack of one sex chromosome in each of the four tissues studied from an infertile mare. Centromere staining of the chromosomes and sex chromatin studies confirmed that this mare had the karyotype 63XO. Clinically this mare was small for her age and breed, exhibited intractable behaviour while at stud and was never observed in oestrus. Her genitalia appeared normal although the uterus and ovaries were small and macroscopic follicles were absent. Histologically, a number of presumptive atretic Graafian follicles and a discrete area of apparently functional luteal tissue were present in the right ovary. The significance of these findings is discussed and compared with the features of analogous syndromes associated with the XO anomaly in other species. It is suggested that the XO anomaly in the horse may not always be associated with complete sterility.