An 8-year-old female Yorkshire Terrier was presented for investigation of reduced appetite, and occasional vomiting. She has been treated with medroxyprogesterone acetate (MPA) from past 3 year-old age for contraception. Abdominal sonography showed abnormal enlargement of uterus, and ovariohysterectomy was performed. Main gross findings of uterus were enlarged lesions in two areas of the left horn, which had thickened wall and yellowish sticky material in the lumen. Histopathologically, cystic endometrial hyperplasia (CEH) and endometritis were present in the thickened area. In this case, CEH and endometritis may be attributed to prolonged treatment of MPA. It was concluded that further study is needed to clarify the association of MPA treatment with age, its pathogenesis and abnormal uterine changes in dogs.