Fluorescence in situ hybridization with the use of the equine X whole chromosome painting probe was carried out on chromosome spreads originating from three mares with poor reproductive performance (infertility, miscarriage or stillbirth). The numbers of analysed spreads were high (105, 300 and 480) and in all three mares a low frequency of mosaicism was identified. The mares had the following karyotypes: 64,XX/63,X/65,XXX (93.6%/5.7%/0.7%), 64,XX/63,X (98.9%/1.1%) and 64,XX/63,X (94.3%/5.7%). The incidence and importance of the low percentage X chromosome mosaicism are discussed.