The ovarian remnant syndrome is a complication of ovariohysterectomy resulting in the presence of functional ovarian tissue in the abdomen. The usual symptom is a return to estrus in a previously ovariohysterectomized bitch or queen. The available methods of diagnosis are vaginal cytologic evaluation during estrus, resting hormone assays, hormone challenge testing, and exploratory laparotomy. Surgical removal of the ovarian remnant is the preferred treatment.