Summary Granulosa cell tumour (GCT) is reported to be the most common ovarian neoplasm in the mare. A multicentre retrospective study evaluating the medical records of horses undergoing ovariectomy for treatment of a GCT was undertaken, documenting the presenting clinicopathological features, treatment and outcome. Follow‐up information was obtained by telephone questionnaire. Fifty‐two mares were included. Most presented with behavioural/reproductive abnormalities, including stallion‐like behaviour (50%), aggression (31%), prolonged oestrus or nymphomania (19%), and persistent anoestrus (8%). Most (93%) affected ovaries had a multilocular appearance when examined ultrasonographically. Only 60% of horses with histopathologically confirmed GCTs demonstrated elevations in serum inhibin concentrations. Twenty‐five percent of horses had histopathologically confirmed bilateral GCTs. Ninety‐four percent of horses that underwent ovariectomy were discharged from the hospital. Standing laparoscopic ovariectomies had a lower complication rate (34%) than other surgical methods. Long‐term complications were uncommon (11%). In conclusion, histopathologically confirmed GCTs can occur in the presence of normal serum inhibin concentrations. The high prevalence of bilateral GCTs indicates that thorough evaluation of the contralateral ovary is necessary. Surgical ovariectomy for treatment of GCTs has favourable short‐ and long‐term outcomes. Standing laparoscopic ovariectomy may be associated with a lower complication rate than other surgical methods.