Testicular feminization syndrome was diagnosed in a mare with aggressive, stallion like behavior and a history of infertility. She was found to have a high baseline testosterone concentration suggesting that testicular tissue was present, and ovarian-like structures examined by use of transrectal ultrasonography had the appearance typical of testicular tissue. Although her external female genitalia appeared normal, her vagina ended in a blind sac, and no cervix or uterus were identified. Surgery was performed, and structures removed from the abdominal cavity were determined to be hypoplastic testicles. Removal of the testicular tissue resulted in complete resolution of her aggressive behavior. Chromosomal evaluation revealed that the mare had 64X,Y (normal male) karyotype. Testicular feminization syndrome is a condition characterized by insensitivity of reproductive tissues to androgens during development because of an abnormality in androgen receptors. This androgen insensitivity results in development of normal external female genitalia, with high testosterone concentrations being released from developing testicles. Testicular feminization syndrome has not been commonly diagnosed in horses, but should be considered as a differential diagnosis for overly aggressive mares with a history of infertility.