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In this case report, an uterus unicornis diagnosed following an operation in a 12 years old sexually intact Cocker Spaniel bitch presented to the clinic at the Faculty of Veterinary Medicine, University of Istanbul with a complaint of prolonged vulval bleeding going on more than 5 weeks has been presented. Extended proestrous was diagnosed based on anamnesis and the clinical findings. An ovariohysterectomy was performed from the mid-line ventral abdominal wall in a routine manner. Visual examination of the reproductive tract during the surgery revealed absence of the left uterine horn, thickness of the right uterine horn and a polycystic left ovary. Histopathological examination of reproductive organs revealed unilateral cornual agenesis and cystic endometrial hyperplasia with an ovarian and para ovarian cysts. In conclusion, despite of regular estrus cycles as in this case, unilateral cornual agenesis should be taken into consideration as a possible hidden reason of infertility in bitches.