Alopecia was the main complaint in a 4 year old female Cocker spaniel dog. bilateral alopecia has been presented on skin lesion for 10 months after ovariohysterectomy about one year ago. Hyperestrogenism was diagnosed by the blood estradiol level test. Left ovarian cyst was resected in the intact ovarian position by laparotomy. Total size of the cyst was about 3 cm and it was the shape of follicular cyst. Blood estradiol level is usually higher than normal in ovarian cyst. But even though estradiol level is in normal range, dermatitis could be induced because the number of estrogen receptor of the skin would be increased or the duration of estrogen secretion would be prolonged. Because the hyperestrogenism in this case is iatrogenic, exact and proper ovariohysterectomy should be made not to induce the complications.