Chromosomal abnormality was detectable in 98 of 180 mares aged 3 years or over with gonadal dysgenesis. The most common abnormality was X monosomy (63,X). The second most common abnormality was a karyotype indistinguishable by G- or C-banding from that of a male horse (64,XY). Two mares demonstrated structural abnormality of one X chromosome [64,X,del(Xp)] which has not previously been reported in horses. One of these foaled a filly with the same karyotype as her dam. Blood typing confirmed parentage of the foal. This is the only example in our experience of fertility in a mare with gonadal dysgenesis and chromosomal abnormality. Chromosomal abnormalities were also found in 4 yearling fillies investigated solely because of small size, poor conformation and lack of vigour. One was 63,X; one was 63,X/64,XX; one was 64X,del(Xp) and the 4th had an autosomal trisomy, tentatively 64,XX,i(?26), which demonstrated a second new abnormal karyotype of the horse.